Methods and materials for controlling the electrochemistry of analyte sensors

ABSTRACT

Embodiments of the invention provide electrochemical analyte sensors having elements designed to modulate their electrochemical reactions as well as methods for making and using such sensors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application which claims the benefitunder 35 U.S.C. §120 and §121 of States patent application Ser. No.11/397,543, filed on Apr. 4, 2006, the contents of which areincorporated by reference. This application is related to U.S. patentapplication Ser. No. 10/273,767 filed Oct. 18, 2002 (published asUS-2004-0074785-A1), U.S. patent application Ser. No. 10/861,837, filedJun. 4, 2004 and U.S. patent application Ser. No. 11/301,512, filed Dec.13, 2005, the contents of each of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to devices and methods for controlling theelectrochemistry of analyte sensors such as glucose sensors used in themanagement of diabetes.

2. Description of Related Art

Electrochemical measurements are widely used to determine theconcentration of specific substances in fluids including biologicalanalytes such as glucose and lactate. Maintaining the appropriateconcentrations of glucose in the blood of an individual for example isextremely important for maintaining homeostasis. A concentration ofglucose below the normal range, or hypoglycemia, can causeunconsciousness and lowered blood pressure, and may even result indeath. A concentration of glucose at levels higher than normal, orhyperglycemia, can result in synthesis of fatty acids and cholesterol,and in diabetics, coma. The measurement of the concentration of glucosein a person's blood, therefore, has become a necessity for diabetics whocontrol the level of blood glucose by insulin therapy.

In clinical settings, accurate and relatively fast determinations ofglucose and/or lactate levels can be determined from blood samplesutilizing electrochemical sensors. In a typical electrochemical sensor,the analyte diffuses from the test environment into the sensor housingthrough a permeable membrane to a working electrode where the analytechemically reacts. A complementary chemical reaction occurs at a secondelectrode in the sensor housing known as a counter electrode. Theelectrochemical sensor produces an analytical signal via the generationof a current arising directly from the oxidation or reduction of theanalyte at the working and counter electrodes. In addition to a workingelectrode and a counter electrode, an electrolytic electrochemicalsensor often includes a third electrode, commonly referred to as areference electrode. A reference electrode is used to maintain theworking electrode at a known voltage or potential.

In general, the electrodes of an electrochemical sensor provide asurface at which an oxidation or a reduction reaction occurs (that is,an electrochemically active surface) to provide a mechanism whereby theionic conduction of an electrolyte solution in contact with theelectrodes is coupled with the electron conduction of each electrode toprovide a complete circuit for a current. By definition, the electrodeat which an oxidation occurs is the anode, while the electrode at whichthe “complementary” reduction occurs is the cathode. In optimal sensors,the working and counter electrode in combination produce an electricalsignal that is both related to the concentration of the analyte and issufficiently strong to provide a signal-to-noise ratio suitable todistinguish between concentration levels of the analyte over the entirerange of interest.

A common type of glucose or lactate electrode sensor comprises an enzymeelectrode which utilizes an enzyme to convert glucose or lactate to anelectroactive product which is then analyzed electrochemically. In suchglucose sensors for example, a chemical reaction at the electrodeconverts glucose in the presence of enzymes, such as glucose oxidase,and results in the formation of reaction products including hydrogenperoxide. In these reactions, glucose reacts with oxygen to formgluconolactone and hydrogen peroxide. A suitable electrode can thenmeasure the formation of hydrogen peroxide as an electrical signal. Thesignal is produced following the transfer of electrons from the peroxideto the electrode, and under suitable conditions, the enzyme catalyzedflow of current is proportional to the glucose concentration. Lactateelectrode sensors including an enzyme electrode, similarly convertlactate in the presence of enzymes, such as lactate oxidase.

With respect to glucose sensors, in typical enzyme electrodes, glucoseand oxygen from blood, as well as some interferants, such as ascorbicacid and uric acid, diffuse through a primary membrane of the sensor. Asthe glucose, oxygen and interferants reach a second membrane, an enzyme,such as glucose oxidase, catalyzes the conversion of glucose to hydrogenperoxide and gluconolactone. The hydrogen peroxide may diffuse backthrough the primary membrane, or it may further diffuse through thesecondary membrane to an electrode where it can be reacted to formoxygen and a proton to produce a current proportional to the glucoseconcentration. While numerous devices for determination of glucose andlactate have been described, most of them have some limitations withrespect to sensitivity, reproducibility, speed of response, number ofeffective uses, and/or the range of detection.

SUMMARY OF THE INVENTION

Embodiments of the invention disclosed herein include electrochemicalanalyte sensors comprising elements such as electrodes and/or electrodecombinations (e.g. working and counter electrode combinations) designedto optimize factors including the reactivity, sensitivity, functioningand lifespan of the analyte sensors. An illustrative embodiment of theinvention is a method of performing an electrochemical reaction withinan analyte sensor comprising using an analyte sensor constructed toinclude an electrode layer configuration that is designed to optimizethe electrochemical reaction at the electrode when the electrode isexposed to an analyte. In such methods the analyte sensor typicallyincludes at least one electrode disposed upon a base substrate wherethis base substrate comprises a geometric feature selected to increasethe surface area of an electrochemically reactive surface of theelectrode disposed thereon such that surface area-to-volume ratio of theelectrochemically-reactive surface area of the electrode disposed on thegeometric feature is greater than surface-area-to-volume ratio of thereactive surface of the electrode when disposed on a flat surface. Insome embodiments of the invention, the surface area to volume ratio ofthe electrochemically reactive surface area of the electrode disposed onthe geometric feature is at least 10%, 25%, 50%, 75% or 100% greaterthan surface area-to-volume ratio of the reactive surface of theelectrode when disposed on a flat surface. In certain embodiments of theinvention, an electrode in the analyte sensor comprises a porous matrix.In such embodiments, the porous matrix can have a surface area that isat least 2, 4, 6, 8, 10, 12, 14, 16 or 18 times the surface area of anessentially non-porous matrix of same dimensions.

Analyte sensors having electrodes constructed to have this type ofconfiguration (e.g. where electrochemically reactive surface area of anelectrode is selected to exhibit a geometry that has anelectrochemically reactive surface area that is greater than if it wereflat) can be constructed by a variety of methods known in the art; forexample, by disposing the electrode material (e.g. a metal such asplatinum) on a base substrate adjoining layer that includes a geometricfeature comprising a lip, a shoulder, a ridge, a notch, a depression, achannel or the like. Typically, the geometric feature of the basesubstrate causes the electrochemically reactive surface area of theelectrode to form a nodular geometry or the like.

In certain embodiments of the invention the analyte sensor comprises aplurality of discrete geometric features having a plurality ofelectrochemically reactive electrode surfaces. Such pluralities offeatures can include patterns such as rows of depressions and/or ridgesor the like; for example, a row of ridges resembling zebra stripes.Optionally, the analyte sensor comprises at least 2, 3, 4, 5, 6, 7, 8, 9or 10 discrete geometric features having a plurality ofelectrochemically reactive electrode surfaces. In a specific embodimentof the invention, analyte sensor further comprises an analyte sensinglayer disposed on the electrode having a relatively high surfacearea-to-volume ratio of the reactive surface, wherein the analytesensing layer detectably alters the electrical current at the electrodein the presence of an analyte; an optional protein layer disposed on theanalyte sensing layer; an adhesion promoting layer disposed on theanalyte sensing layer or the optional protein layer, wherein theadhesion promoting layer promotes the adhesion between the analytesensing layer and an analyte modulating layer disposed on the analytesensing layer; an analyte modulating layer disposed on the analytesensing layer, wherein the analyte modulating layer modulates thediffusion of the analyte therethrough; and an optional cover layerdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture over at least a portion ofthe analyte modulating layer. In certain embodiments of the invention,the analyte sensor is designed to be implantable within a mammal.Optionally the electrochemical reaction at the electrode involves aprotein reactive with an analyte present in mammalian blood such asglucose oxidase, glucose dehydrogenase, lactate oxidase, hexokinase orlactate dehydrogenase.

A related embodiment of the invention is an analyte sensor for detectingan analyte in a fluid, the apparatus comprising at least one electrodedisposed upon a base substrate, wherein the base substrate includes ageometric feature selected to increase the surface area of anelectrochemically reactive surface on the electrode deposited thereon(e.g. a lip, a shoulder, a ridge, a notch, a depression, a channel orthe like) such that surface-area-to-volume ratio of theelectrochemically reactive surface area of the electrode disposed on thegeometric feature is greater than surface area-to-volume ratio of thereactive surface of the electrode when disposed on a flat surface.Optionally, the analyte sensor comprises a plurality of discretegeometric features having a plurality of electrochemically reactiveelectrode surfaces. Typically, the analyte sensor is implantable andcomprises an analyte sensing layer disposed on the electrode, whereinthe analyte sensing layer detectably alters the electrical current atthe electrode in the presence of an analyte; an optional protein layerdisposed on the analyte sensing layer; an adhesion promoting layerdisposed on the analyte sensing layer or the optional protein layer,wherein the adhesion promoting layer promotes the adhesion between theanalyte sensing layer and an analyte modulating layer disposed on theanalyte sensing layer; and an analyte modulating layer disposed on theanalyte sensing layer, wherein the analyte modulating layer modulatesthe diffusion of the analyte therethrough; and an optional cover layerdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture over at least a portion ofthe analyte modulating layer. In certain embodiments of the invention,the implantable analyte sensor further comprises an interferencerejection layer disposed between the surface of the working electrodeand the analyte sensing layer.

Yet another embodiment of the invention is a method of modulatingelectrochemical reactions within an implantable analyte sensor, themethod comprising performing electrochemical reactions within animplantable analyte sensor comprising a working electrode having areactive surface area, wherein during analyte sensing, the workingelectrode generates electrons that reduce a plurality of compositionspecies in the electrochemical reaction including oxygen (O₂); and acounter electrode having a reactive surface area, wherein the size ofthe reactive surface area of the counter electrode is selected so as tocontrol the reduction of the plurality of composition species in theelectrochemical reaction so that oxygen (O₂) is the predominantcomposition species reduced by the electrons generated at the workingelectrode, wherein the compound substrates having a first affinity forthe electrons exhibit an affinity that is higher than the affinity ofthe compound substrates having a second affinity for the electrons; anda counter electrode of a second surface area, the second surface area isselected to be a size that reduces the interaction between electronsgenerated at the working electrode with compound substrates having thesecond affinity for the electrons generated at the working electrode; sothat electrochemical reactions within the implantable analyte sensor aremodulated. In such methods the surface area of the counter electrode istypically about 1.5, 2, 2.5 or 3 times the size of the workingelectrode.

A related embodiment of the invention is an implantable electrochemicalanalyte sensor designed to include this electrode architecture.Optionally, the working electrode and the counter electrode in theanalyte sensor comprise a porous matrix. Alternatively, the workingelectrode can comprise a relatively nonporous matrix while the counterelectrode can comprise a porous matrix or vice versa. Optionally theporous matrix has a surface area that is at least 1.5, 2, 2.5, 3, 3.5,4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 10, 12, 14, 16 or 18 times thesurface area of an essentially non-porous matrix of same dimensions.Optionally, the implantable analyte sensor further comprises an analytesensing layer disposed on the working electrode, wherein the analytesensing layer detectably alters the electrical current at the workingelectrode in the presence of an analyte; an optional protein layerdisposed on the analyte sensing layer; an adhesion promoting layerdisposed on the analyte sensing layer or the optional protein layer,wherein the adhesion promoting layer promotes the adhesion between theanalyte sensing layer and an analyte modulating layer disposed on theanalyte sensing layer; and an analyte modulating layer disposed on theanalyte sensing layer, wherein the analyte modulating layer modulatesthe diffusion of the analyte therethrough; and an optional cover layerdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture over at least a portion ofthe analyte modulating layer.

Yet another illustrative embodiment of the invention is a method ofmaking a metallic electrode by electrodepositing a plurality of metallayers that comprise the electrode using cycles of differingelectroplating conditions. Typically, the method comprises a first cycleof electroplating where a metal is electrodeposited onto a substrateunder a first set of conditions selected to produce a first metal layerhaving a first surface area and a first adhesion strength between thesubstrate and the first metal layer. The method then involves a secondcycle of electroplating where a metal composition is thenelectrodeposited onto the first metal layer under a second set ofconditions selected to produce a second metal layer having a secondsurface area and a second adhesion strength between the first metallayer and the second metal layer. In this method, the first and secondset of conditions are selected to produce a second metal layer having asecond surface area that is greater than the first surface area of thefirst metal layer produced by the first set of conditions and a secondmetal layer having an adhesion with the first metal layer that isgreater than the adhesion between the first metal layer and thesubstrate produced by the first set of conditions. Optionally, themethod further comprises additional cycles of electroplating. In onesuch example, the method comprises a third cycle of electroplating wherea metal composition is electrodeposited onto the second layer under athird set of conditions selected to produce a third metal layer having athird surface area. Typically, the second and third set of conditionsare selected to produce a third metal layer having a greater densitythan the density of the second metal layer. In certain embodiments ofthe invention, the third set of conditions produces a third metal layerhaving a third surface area that is less than the second surface area ofthe second metal layer produced by the second set of conditions.

The invention also provides additional articles of manufacture includingsensor elements, sensor sets and kits. In one such embodiment of theinvention, a kit and/or sensor element or set, useful for the sensing ananalyte as is described above, is provided. The kit and/or sensor settypically comprises a container, a label and a sensor as describedabove. The typical embodiment is a kit comprising a container and,within the container, an analyte sensor apparatus having a design asdisclosed herein and instructions for using the analyte sensorapparatus.

Other objects, features and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddescription. It is to be understood, however, that the detaileddescription and specific examples, while indicating some embodiments ofthe present invention are given by way of illustration and notlimitation. Many changes and modifications within the scope of thepresent invention may be made without departing from the spirit thereof,and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides a schematic of the well known reaction between glucoseand glucose oxidase. As shown in a stepwise manner, this reactioninvolves glucose oxidase (GOx), glucose and oxygen in water. In thereductive half of the reaction, two protons and electrons aretransferred from β-D-glucose to the enzyme yielding d-gluconolactone. Inthe oxidative half of the reaction, the enzyme is oxidized by molecularoxygen yielding hydrogen peroxide. The d-gluconolactone then reacts withwater to hydrolyze the lactone ring and produce gluconic acid. Incertain electrochemical sensors of the invention, the hydrogen peroxideproduced by this reaction is oxidized at the working electrode (H₂O₂→2H++O₂+2e⁻).

FIG. 2 provides a diagrammatic view of a typical analyte sensorconfiguration of the current invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Unless otherwise defined, all terms of art, notations and otherscientific terms or terminology used herein are intended to have themeanings commonly understood by those of skill in the art to which thisinvention pertains. In some cases, terms with commonly understoodmeanings are defined herein for clarity and/or for ready reference, andthe inclusion of such definitions herein should not necessarily beconstrued to represent a substantial difference over what is generallyunderstood in the art. Many of the techniques and procedures describedor referenced herein are well understood and commonly employed usingconventional methodology by those skilled in the art. As appropriate,procedures involving the use of commercially available kits and reagentsare generally carried out in accordance with manufacturer definedprotocols and/or parameters unless otherwise noted. A number of termsare defined below.

The term “analyte” as used herein is a broad term and is used in itsordinary sense, including, without limitation, to refer to a substanceor chemical constituent in a fluid such as a biological fluid (forexample, blood, interstitial fluid, cerebral spinal fluid, lymph fluidor urine) that can be analyzed. Analytes can include naturally occurringsubstances, artificial substances, metabolites, and/or reactionproducts. In some embodiments, the analyte for measurement by thesensing regions, devices, and methods is glucose. However, otheranalytes are contemplated as well, including but not limited to,lactate. Salts, sugars, proteins fats, vitamins and hormones naturallyoccurring in blood or interstitial fluids can constitute analytes incertain embodiments. The analyte can be naturally present in thebiological fluid or endogenous; for example, a metabolic product, ahormone, an antigen, an antibody, and the like. Alternatively, theanalyte can be introduced into the body or exogenous, for example, acontrast agent for imaging, a radioisotope, a chemical agent, afluorocarbon-based synthetic blood, or a drug or pharmaceuticalcomposition, including but not limited to insulin. The metabolicproducts of drugs and pharmaceutical compositions are also contemplatedanalytes.

The term “sensor,” as used herein, is a broad term and is used in itsordinary sense, including, without limitation, the portion or portionsof an analyte-monitoring device that detects an analyte. In oneembodiment, the sensor includes an electrochemical cell that has aworking electrode, a reference electrode, and optionally a counterelectrode passing through and secured within the sensor body forming anelectrochemically reactive surface at one location on the body, anelectronic connection at another location on the body, and a membranesystem affixed to the body and covering the electrochemically reactivesurface. During general operation of the sensor, a biological sample(for example, blood or interstitial fluid), or a portion thereof,contacts (directly or after passage through one or more membranes ordomains) an enzyme (for example, glucose oxidase); the reaction of thebiological sample (or portion thereof) results in the formation ofreaction products that allow a determination of the analyte level in thebiological sample.

The term “electrochemical cell,” as used herein, is a broad term and isused in its ordinary sense, including, without limitation, a device inwhich chemical energy is converted to electrical energy. Such a celltypically consists of two or more electrodes held apart from each otherand in contact with an electrolyte solution. Connection of theelectrodes to a source of direct electric current renders one of themnegatively charged and the other positively charged. Positive ions inthe electrolyte migrate to the negative electrode (cathode) and therecombine with one or more electrons, losing part or all of their chargeand becoming new ions having lower charge or neutral atoms or molecules;at the same time, negative ions migrate to the positive electrode(anode) and transfer one or more electrons to it, also becoming new ionsor neutral particles. The overall effect of the two processes is thetransfer of electrons from the negative ions to the positive ions, achemical reaction.

The terms “electrochemically reactive surface” and “electroactivesurface” as used herein are broad terms and are used in their ordinarysense, including, without limitation, the surface of an electrode wherean electrochemical reaction takes place. In one example, a workingelectrode measures hydrogen peroxide produced by the enzyme catalyzedreaction of the analyte being detected reacts creating an electriccurrent (for example, detection of glucose analyte utilizing glucoseoxidase produces H₂O₂ as a by product, H₂O₂ reacts with the surface ofthe working electrode producing two protons (2H⁺), two electrons (2e⁻)and one molecule of oxygen (O₂) which produces the electronic currentbeing detected). In the case of the counter electrode, a reduciblespecies, for example, O₂ is reduced at the electrode surface in order tobalance the current being generated by the working electrode.

The term “sensing region” as used herein is a broad term and is used inits ordinary sense, including, without limitation, the region of amonitoring device responsible for the detection of a particular analyte.In an illustrative embodiment, the sensing region can comprise anon-conductive body, a working electrode, a reference electrode, and acounter electrode passing through and secured within the body formingelectrochemically reactive surfaces on the body and an electronicconnective means at another location on the body, and a one or morelayers covering the electrochemically reactive surface.

The terms “electrical potential” and “potential” as used herein, arebroad terms and are used in their ordinary sense, including, withoutlimitation, the electrical potential difference between two points in acircuit which is the cause of the flow of a current. The term “systemnoise,” as used herein, is a broad term and is used in its ordinarysense, including, without limitation, unwanted electronic ordiffusion-related noise which can include Gaussian, motion-related,flicker, kinetic, or other white noise, for example.

The terms “interferants” and “interfering species,” as used herein, arebroad terms and are used in their ordinary sense, including, but notlimited to, effects and/or species that interfere with the measurementof an analyte of interest in a sensor to produce a signal that does notaccurately represent the analyte measurement. In one example of anelectrochemical sensor, interfering species are compounds with anoxidation potential that overlaps with the analyte to be measured.

As discussed in detail below, embodiments of the invention relate to theuse of an electrochemical sensor that measures a concentration of ananalyte of interest or a substance indicative of the concentration orpresence of the analyte in fluid. In some embodiments, the sensor is acontinuous device, for example a subcutaneous, transdermal, orintravascular device. In some embodiments, the device can analyze aplurality of intermittent blood samples. The sensor embodimentsdisclosed herein can use any known method, including invasive, minimallyinvasive, and non-invasive sensing techniques, to provide an outputsignal indicative of the concentration of the analyte of interest.Typically, the sensor is of the type that senses a product or reactantof an enzymatic reaction between an analyte and an enzyme in thepresence of oxygen as a measure of the analyte in vivo or in vitro. Suchsensors typically comprise a membrane surrounding the enzyme throughwhich a bodily fluid passes and in which an analyte within the bodilyfluid reacts with an enzyme in the presence of oxygen to generate aproduct. The product is then measured using electrochemical methods andthus the output of an electrode system functions as a measure of theanalyte. In some embodiments, the sensor can use an amperometric,coulometric, conductimetric, and/or potentiometric technique formeasuring the analyte.

Embodiments of the invention described herein can be adapted andimplemented with a wide variety of known electrochemical sensors,including for example, U.S. Patent Application No. 20050115832, U.S.Pat. Nos. 6,001,067, 6,702,857, 6,212,416, 6,119,028, 6,400,974,6,595,919, 6,141,573, 6,122,536, 6,512,939 5,605,152, 4,431,004,4,703,756, 6,514,718, 5,985,129, 5,390,691, 5,391,250, 5,482,473,5,299,571, 5,568,806, 5,494,562, 6,120,676, 6,542,765 as well as PCTInternational Publication Numbers WO 01/58348, WO 04/021877, WO03/034902, WO 03/035117, WO 03/035891, WO 03/023388, WO 03/022128, WO03/022352, WO 03/023708, WO 03/036255, WO03/036310 and WO 03/074107, andEuropean Patent Application EP 1153571, the contents of each of whichare incorporated herein by reference.

Embodiments of the invention disclosed herein provide sensors of thetype used, for example, in subcutaneous or transcutaneous monitoring ofblood glucose levels in a diabetic patient. A variety of implantable,electrochemical biosensors have been developed for the treatment ofdiabetes and other life-threatening diseases. Many existing sensordesigns use some form of immobilized enzyme to achieve theirbio-specificity. For example, a first class of glucose sensor designsuse a very thin (<1 micron) layer of glucose oxidase (GOx) and bovineserum albumin that is either spray or spin coated onto the workingelectrode and cross-linked with glutaraldehyde. Alternatively, a secondclass of glucose sensor design employs a thick (˜1 mm) hydrogel known asthe Sensor Matrix Protein (SMP), which typically consists of an enzymesuch as GOx and human serum albumin cross-linked together with across-linking agent such as glutaraldehyde. Relative to each other, theimmobilized enzyme configurations of the two above-noted classes ofsensor designs possess different advantages that serve to increaseoperational sensor life. Due to the close proximity of the immobilizedGOx to the peroxide-consuming electrode, the first class of sensordesigns are believed to possess significantly decreased enzymedeactivation rate constants. In comparison, the thick SMPs utilized inthe second class of sensor designs can incorporate orders of magnitudemore enzyme than the first class.

Many sensor designs utilize a matrix (or a plurality of matrices) suchas an enzymatic hydrogel matrix to function. The term “matrix” is usedherein according to its art-accepted meaning of something within or fromwhich something else originates, develops, takes form and/or is found.An exemplary enzymatic hydrogel matrix for example typically comprises abio-sensing enzyme (e.g. glucose oxidase or lactate oxidase) and humanserum albumin proteins that have been cross-linked together with acrosslinking agent such as glutaraldehyde to form a polymer network.This network is then swollen with an aqueous solution to form anenzymatic hydrogel matrix. The degree of swelling of this hydrogelfrequently increases over a time-period of several weeks, and ispresumably due to the degradation of network cross-links. Regardless ofits cause, an observed consequence of this swelling is the protrusion ofthe hydrogel outside of the hole or “window” cut into the outer sensortubing. This causes the sensor dimensions to exceed designspecifications and has a negative impact on its analytical performance.

Embodiments of the invention disclosed herein provide sensor elementshaving enhanced material properties and sensors constructed from suchelements. The disclosure further provides methods for making and usingsuch sensors. While some embodiments of the invention pertain to glucoseand/or lactate sensors, a variety of the elements disclosed herein (e.g.electrodes and electrode designs) can be adapted for use with any one ofthe wide variety of sensors known in the art. The analyte sensorelements, architectures and methods for making and using these elementsthat are disclosed herein can be used to establish a variety of layeredsensor structures. Such sensors of the invention exhibit a surprisingdegree of flexibility and versatility, characteristic which allow a widevariety of sensor configurations to be designed to examine a widevariety of analyte species.

In typical embodiments of the present invention, the transduction of theanalyte concentration into a processable signal is by electrochemicalmeans. These transducers may include any of a wide variety ofamperometric, potentiometric, or conductimetric base sensors known inthe art. Moreover, the microfabrication sensor techniques and materialsof the instant invention may be applied to other types of transducers(e.g., acoustic wave sensing devices, thermistors, gas-sensingelectrodes, field-effect transistors, optical and evanescent field waveguides, and the like) fabricated in a substantially nonplanar, oralternatively, a substantially planar manner. A useful discussion andtabulation of transducers which may be exploited in a biosensor as wellas the kinds of analytical applications in which each type of transduceror biosensor, in general, may be utilized, is found in an article byChristopher R. Lowe in Trends in Biotech. 1984, 2(3), 59-65.

Specific aspects of the invention are discussed in detail in thefollowing sections.

I. Typical Elements, Configurations and Analyte Sensors of the InventionA. Optimized Sensor Elements of the Invention

Embodiments of the sensors disclosed herein incorporate one or moresensor elements having enhanced material properties. Embodiments of theinvention include sensor including these elements and well methods formaking and using them. Embodiments of the invention disclosed hereininclude electrochemical analyte sensors comprising elements such aselectrodes and/or electrode combinations (e.g. working an counterelectrode combinations) designed to optimize factors including thereactivity, sensitivity, functioning and lifespan of the analytesensors. Certain specific embodiments of the invention are designed tooptimize the electrochemical reactions that function in the sensing ofan analyte of interest. The optimized embodiments of the inventiondisclosed herein can be utilized and/or applied to a wide variety ofsensor methods and designs. The following sections describe illustrativesensor elements, sensor configurations and methodological embodiments ofthe invention.

Certain embodiments of the invention are designed to enhance sensorstability. As discussed in detail below, embodiments of the inventioninclude analyte sensors having a plurality of adjoining layersconsisting of different functional constituents. As is known in the art,with certain sensors that include a plurality of layers, one or more ofthe layers can become unstable and separate in part or in whole from anadjoining layer. Such delamination events can compromise the function ofone or more of the different constituents and consequently have anegative impact on a sensor's function, for example its long termstability. In this context, we observe that, with certain sensorembodiments a series of flat, uniform layers typically have betteradherence between layers and are therefore more stable than sensorshaving disparate layers of rough and/or polymorphic constituents.

One example of layer that can exhibit problems with delamination is anelectrode layer that is deposited upon a base layer in a manner thatcreates an irregular surface, for example when a platinum electrodelayer is deposited on a base substrate that has an edge or shoulder thatcauses a nodule of a electrode material (e.g. platinum black) to formover this feature on the base substrate. This can happen, for example,when platinum black is disposed in the sensor in a manner that allows itto grow up the side of a material already in the sensor; for example, anedge of an insulating composition. In this context, one embodiment ofthe invention is a method of contributing to the uniformity of a layerin an analyte sensing device comprising a plurality of layers, themethod comprising disposing an electrode used in the device on a basesubstrate, wherein the base substrate is selected to exhibit anessentially or predominantly flat geometry so that the electrodedisposed on the base substrate also exhibits a relatively flat geometry,thereby contributing to the uniformity of a layer in a analyte sensingdevice. In this method, the sensor can be manufactured according to aprocess that avoids depositing the electrode materials on geometricfeatures already present in the sensor. Optionally, the base substrateis selected to exhibit a flat geometry by avoiding disposing theelectrode on a ridge, lip, shoulder or edge generated by an insulatingcomposition that is disposed in an area where the electrode is disposed.

A related embodiment of the invention is a method of contributing to theuniformity of a layer in an analyte sensing device comprising aplurality of layers, the method including disposing an electrode used inthe device on a base substrate, and then removing a portion of theelectrode so disposed on the base substrate, wherein the portion of theelectrode disposed on the base substrate that is removed is an irregularfeature such as a bulge or the like that results from the electrodebeing disposed on a lip, shoulder, edge or the like etc. that is presentin another of the plurality of layers that is disposed in an area wherethe electrode is disposed. In this method, the resulting layer havingthe electrode where the bulge or nodule is removed exhibits a moreuniform surface than does the layer where the bulge is not removed. Suchbulges can be removed by one of a variety of methods known in the art,for example, via mechanical or chemical processes. Typically, thesemethods of contributing to the uniformity of one or more layers in ananalyte sensor have the result of inhibiting delamination of a layerwithin the plurality of layers. These methods of contributing to theuniformity of one or more layers in an analyte sensor can also increasesensor stability; for example, the in vivo stability of a sensor.

In view of the reasons noted above, those of skill in this art familiarwith this phenomena would be motivated to construct sensors having aplurality of uniform layers. However, as disclosed herein, it has beendiscovered that in certain contexts, sensors having electrode surfaceswith unusual and/or non-uniform geometries have unexpected and desirableproperties. In particular, sensors designed so that the electrodematerial (e.g. platinum) is deposited on a base substrate that includesa geometric feature selected to increase the surface area of anelectrochemically reactive surface on the electrode exhibit a number ofbeneficial properties, such as the ability to generate a greater signalin response to analyte, a reduced signal to noise ratio and a greaterlongevity, in particular, a greater longevity in vivo. Surprisingly, incertain embodiments of the invention, the beneficial properties of theseshaped electrodes outweigh the detrimental effects that result from alack of uniformity in the electrode layer. This unexpected observationforms the basis of certain embodiments of the invention discussed below.

One illustrative embodiment of the invention is a method of performingan electrochemical reaction within an analyte sensor comprising using ananalyte sensor constructed to include an electrode layer configurationthat is designed to optimize the electrochemical reaction at theelectrode when the electrode is exposed to an analyte. In such methodsthe analyte sensor typically includes at least one electrode disposedupon a base substrate where this base substrate comprises a geometricfeature selected to increase the surface area of an electrochemicallyreactive surface of the electrode disposed thereon such that the surfacearea-to-volume ratio of the electrochemically reactive surface area ofthe electrode disposed on the geometric feature is greater than surfacearea-to-volume ratio of the reactive surface of the electrode whendisposed on a flat surface. Optionally, the electrode further comprisesa porous matrix. In certain embodiments, the porous matrix has a surfacearea that is at least 2, 4, 6, 8, 10, 12, 14, 16 or 18 times the surfacearea of an essentially non-porous matrix of same dimensions.

In certain methodological embodiments of the invention, exposing theanalyte sensor to an analyte so that an electrochemical reaction isperformed within an analyte sensor having electrodes constructed to havethis type of configuration, the electronic signal in response toexposure to the analyte that is generated at the electrochemicallyreactive surface area of the electrode disposed on the geometric featureis greater than the electronic signal generated an electrochemicallyreactive surface area of the electrode when the electrode is disposed ona flat surface. Similarly, in certain methodological embodiments of theinvention, exposing the analyte sensor to an analyte so that anelectrochemical reaction is performed within an analyte sensor havingelectrodes constructed to have this type of configuration, the in vivolifetime of the analyte sensor having the electrochemically reactivesurface area of the electrode disposed on the geometric feature isgreater than the in vivo lifetime of an analyte sensor having anelectronic signal generated on an electrochemically reactive surfacearea of an electrode when the electrode is disposed on a flat surface.

While not being bound by a specific scientific theory, it is believedthat the highly desirable properties that are observed with analytesensors constructed to have more complex geometries are due to arelative increase in the size of the electrochemically reactive surfacearea of these electrodes. In particular, embodiments of the inventionusing such electrode geometries are believed to generate a largerelectrochemically reactive surface area for reacting with and sensinganalytes while the amount of total space occupied by the electrode layerwithin the sensor remains essentially unchanged or changes very little.This allows for a large surface area in a small space. A similarphenomena is observed in human brains, organs that are highly folded togenerate a large surface area within a small space. In particular, thecerebral cortex of the human brain is highly convoluted, meaning it hasmany folds and creases, convolutions that allow a large surface area ofbrain (and an associated larger number of neurons) to fit inside ourrelatively small skulls. In some embodiments of the invention, thesurface area to volume ratio of the electrochemically reactive surfacearea of the electrode disposed on the geometric feature is at least 10%,25%, 50%, 75% or 100% greater than surface area-to-volume ratio of thereactive surface of the electrode when disposed on a flat surface.

Analyte sensor having electrodes constructed to have this type ofconfiguration (e.g. where electrochemically reactive surface area of anelectrode is disposed on the geometric feature so that theelectrochemically reactive surface area is greater than if it wasdisposed on a flat surface) can be constructed by a variety of methodsknown in the art, for example by disposing the electrode material (e.g.a metal such as platinum) on a base substrate adjoining layer thatincludes a geometric feature comprising a lip, a shoulder, a ridge, anotch, a depression, a channel or the like. Typically, the geometricfeature of the base substrate causes the electrochemically reactivesurface area of the electrode to form a nodule or the like. While thedeposition of an electrode onto a base substrate is one way to generatean electrode with a high surface area to volume ratio, electrodes havingthese properties can be generated by other processes known in the art.For example, in an alternative embodiment of the invention, electrodeshaving a high surface area to volume ratio can be premade and thensubsequently disposed within the analyte sensor.

In certain embodiments of the invention the analyte sensor comprises aplurality of discrete geometric features having a plurality ofelectrochemically reactive electrode surfaces. Such pluralities offeatures can include patterns such as rows of depressions and/or ridgesor the like, for example a row of ridges resembling zebra stripes. Insome embodiments of the invention, the sensor is manufactured to includethese pluralities of features on which the electrode material can bedeposited. Optionally, the analyte sensor comprises at least 2, 3, 4, 5,6, 7, 8, 9 or 10 discrete geometric features having a plurality ofelectrochemically reactive electrode surfaces. In a specific embodimentof the invention, analyte sensor further comprises an analyte sensinglayer disposed on the electrode having a relatively high surfacearea-to-volume ratio of the reactive surface, wherein the analytesensing layer detectably alters the electrical current at the electrodein the presence of an analyte; an optional protein layer disposed on theanalyte sensing layer; an adhesion promoting layer disposed on theanalyte sensing layer or the optional protein layer, wherein theadhesion promoting layer promotes the adhesion between the analytesensing layer and an analyte modulating layer disposed on the analytesensing layer; and an analyte modulating layer disposed on the analytesensing layer, wherein the analyte modulating layer modulates thediffusion of the analyte therethrough; and an optional cover layerdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture over at least a portion ofthe analyte modulating layer. In certain embodiments of the invention,the analyte sensor is designed to be implantable within a mammal.Optionally the electrochemical reaction at the electrode involves aprotein reactive with an analyte present in mammalian blood such asglucose oxidase, glucose dehydrogenase, lactate oxidase, hexokinase orlactate dehydrogenase. In some embodiments of the invention, in theelectrochemical reaction, hydrogen peroxide is oxidized at theelectrochemically reactive surface area of the electrode disposed on thegeometric feature of the base substrate.

A related embodiment of the invention is an analyte sensor for detectingan analyte in a fluid, the apparatus comprising at least one electrodedisposed upon a base substrate, wherein the base substrate includes ageometric feature selected to increase the surface area of anelectrochemically reactive surface on the electrode deposited thereon(e.g. a lip, a shoulder, a ridge, a notch, a depression, a channel orthe like) such that surface area-to-volume ratio of theelectrochemically reactive surface area of the electrode disposed on thegeometric feature is greater than surface area-to-volume ratio of thereactive surface of the electrode when disposed on a flat surface.Optionally, the analyte sensor comprises a plurality of discretegeometric features having a plurality of electrochemically reactiveelectrode surfaces. Typically, the analyte sensor is implantable andcomprises an analyte sensing layer disposed on the electrode, whereinthe analyte sensing layer detectably alters the electrical current atthe electrode in the presence of an analyte; an optional protein layerdisposed on the analyte sensing layer; an adhesion promoting layerdisposed on the analyte sensing layer or the optional protein layer,wherein the adhesion promoting layer promotes the adhesion between theanalyte sensing layer and an analyte modulating layer disposed on theanalyte sensing layer; and an analyte modulating layer disposed on theanalyte sensing layer, wherein the analyte modulating layer modulatesthe diffusion of the analyte therethrough; and an optional cover layerdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture over at least a portion ofthe analyte modulating layer. In certain embodiments of the invention,the implantable analyte sensor further comprises an interferencerejection layer disposed between the surface of the working electrodeand the analyte sensing layer.

Yet another embodiment of the invention is a method of modulatingelectrochemical reactions within an implantable analyte sensor, themethod comprising performing electrochemical reactions within animplantable analyte sensor comprising: a working electrode having afirst surface area, wherein electrochemical reactions at the workingelectrode generate electrons that interact with compound substrateshaving a first affinity for the electrons that is greater than or equalto that of oxygen (O₂), and compound substrates having a second affinityfor the electrons that is less than or equal to that of oxygen (O₂),wherein the second surface area is selected to be a size that reducesthe interaction between electrons generated at the working electrodewith compound substrates having the second affinity for the electronsgenerated at the working electrode; so that electrochemical reactionswithin the implantable analyte sensor are modulated.

A closely related embodiment of the invention is a method of modulatingelectrochemical reactions within an implantable analyte sensor, themethod comprising performing electrochemical reactions within animplantable analyte sensor comprising: a working electrode having areactive surface area, wherein during analyte sensing, the workingelectrode generates electrons that reduce a plurality of compositionspecies in the electrochemical reaction including oxygen (O₂); and acounter electrode having a reactive surface area, wherein the size ofthe reactive surface area of the counter electrode (e.g. relative to thesize of the reactive surface area of the working electrode) is selectedso as to control the reduction of the plurality of composition speciesin the electrochemical reaction so that oxygen (O₂) is the predominantcomposition species reduced by the electrons generated at the workingelectrode over the functional lifetime of the sensor, so thatelectrochemical reactions within the implantable analyte sensor aremodulated over the lifetime of the sensor. The term predominant is usedherein according to its art accepted meaning of being the most frequentor common. In one illustrative example of such an embodiment, 51% of theelectrons generated at the working electrode reduce oxygen (O₂) speciesover the lifetime of the sensor.

The methods (and associated sensor architectures) designed toselectively reduce oxygen (O₂) species in the electrochemical reactionshave a number of surprising advantages over existing embodiments in theart. By constructing sensors with elements specifically selected so thatthe size of the reactive surface area of the counter electrode relativeto the size of the reactive surface area of the working electrodecontrols the reduction of the plurality of composition species, thesensing of the analyte is therefore occurs in a controlled environment.In this way, this sensor structure maintains a greater consistency inthe electrochemical reaction environment (leading to a greaterconsistency in signals generated by the electrochemical reactions)during the period that the analyte sensor is in use. For example, whenthe analyte sensors employ a oxidase in analyte sensing such as theprotein glucose oxidase, using this sensor structure to control theelectrochemical reaction so that oxygen (O₂) is the predominantcomposition species reduced by the electrons generated at the workingelectrode over the functional lifetime of the sensor provides a benefitby providing a stabilized reactive environment for the electrochemicalreactions that function to provide the sensor signal.

While analyte sensors having working and counter electrodes of differentsizes may be described in the art, the art fails to teach or suggest theinvention described herein, e.g. sensors constructed to include elementsspecifically designed to control the electrochemical reactions in thedescribed manner (i.e. so that oxygen (O₂) is the predominantcomposition species reduced by electrons generated at the workingelectrode). In addition, embodiments of the sensors disclosed herein aredesigned to vary the size of the reactive surface area of the counterelectrode relative to the size of the reactive surface area of theworking electrode, and not just the size of the electrodes. For example,in certain embodiments of the invention, the counter and workingelectrodes can be of approximately the same size. In such embodiments,the counter electrode can be constructed to exhibit an architecture thatprovides a greater surface area/size ratio (e.g. one constructed using aporous matrix) so that the greater size of the reactive surface area ofthe counter electrode (e.g. relative to the size of the reactive surfacearea of the working electrode) controls the reduction of the pluralityof composition species in the electrochemical reaction so that oxygen(O₂) is the predominant composition species reduced by the electronsgenerated at the working electrode.

Optionally, the working electrode and the counter electrode in analytesensors having the above-noted architectures comprise a porous matrix.Alternatively, the working electrode comprises a relatively nonporousmatrix while the counter electrode comprises a porous matrix or viceversa. Optionally the porous matrix has a surface area that is at least2, 4, 6, 8, 10, 12, 14, 16 or 18 times the surface area of anessentially non-porous matrix of same dimensions. In such methods thesurface area of the counter electrode is typically about 1.5, 2, 2.5 or3 times the size of the working electrode. Optionally, the implantableanalyte sensor comprises an analyte sensing layer disposed on theworking electrode, wherein the analyte sensing layer detectably altersthe electrical current at the working electrode in the presence of ananalyte; an optional protein layer disposed on the analyte sensinglayer; an adhesion promoting layer disposed on the analyte sensing layeror the optional protein layer, wherein the adhesion promoting layerpromotes the adhesion between the analyte sensing layer and an analytemodulating layer disposed on the analyte sensing layer; and an analytemodulating layer disposed on the analyte sensing layer, wherein theanalyte modulating layer modulates the diffusion of the analytetherethrough; and an optional cover layer disposed on at least a portionof the analyte modulating layer, wherein the cover layer furtherincludes an aperture over at least a portion of the analyte modulatinglayer.

A related embodiment of the invention is an implantable electrochemicalanalyte sensor comprising a working electrode having a first surfacearea, wherein electrochemical reactions at the working electrodegenerate electrons that interact with compound substrates having a firstaffinity for the electrons and compound substrates having a secondaffinity for the electrons, wherein the compound substrates having afirst affinity for the electrons exhibit an affinity that is higher thanthe affinity of the compound substrates having a second affinity for theelectrons; and a counter electrode of a second surface area, the secondsurface area is selected to be a size that reduces the interactionbetween electrons generated at the working electrode with compoundsubstrates having the second affinity for the electrons generated at theworking electrode; an analyte sensing layer disposed on the workingelectrode, wherein the analyte sensing layer detectably alters theelectrical current at the working electrode in the presence of ananalyte; an adhesion promoting layer disposed on the analyte sensinglayer or the protein layer, wherein the adhesion promoting layerpromotes the adhesion between the analyte sensing layer and an analytemodulating layer disposed on the analyte sensing layer; an analytemodulating layer disposed on the analyte sensing layer, wherein theanalyte modulating layer modulates the diffusion of the analytetherethrough; and a cover layer disposed on at least a portion of theanalyte modulating layer, wherein the cover layer further includes anaperture over at least a portion of the analyte modulating layer.

B. Diagrammatic Illustration of Typical Sensor Configurations

FIG. 2 illustrates a cross-section of a typical sensor structure 100 ofthe present invention. The sensor is formed from a plurality ofcomponents that are typically in the form of layers of variousconductive and non-conductive constituents disposed on each otheraccording to a method of the invention to produce a sensor structure.The components of the sensor are typically characterized herein aslayers because, for example, it allows for a facile characterization ofthe sensor structure shown in FIG. 2. Artisans will understand however,that in certain embodiments of the invention, the sensor constituentsare combined such that multiple constituents form one or moreheterogeneous layers. In this context, those of skill in the artunderstand that the ordering of the layered constituents can be alteredin various embodiments of the invention.

The embodiment shown in FIG. 2 includes a base layer 102 to support thesensor 100. The base layer 102 can be made of a material such as a metaland/or a ceramic and/or a polymeric substrate, which may beself-supporting or further supported by another material as is known inthe art. Embodiments of the invention include a conductive layer 104which is disposed on and/or combined with the base layer 102. In certainembodiments, the base layer 102 and/or the conductive layer 104 can beconstructed to produce electrodes having a configuration where theelectrochemically reactive surface area of an electrode is disposed onthe geometric feature so that the electrochemically reactive surfacearea is greater than if it was disposed on a flat surface.

Typically the conductive layer 104 comprises one or more electrodes. Anoperating sensor 100 typically includes a plurality of electrodes suchas a working electrode, a counter electrode and a reference electrode.Other embodiments may also include an electrode that performs multiplefunctions, for example one that functions as both as a reference and acounter electrode. Still other embodiments may utilize a separatereference element not formed on the sensor. Typically these electrodesare electrically isolated from each other, while situated in closeproximity to one another.

As discussed in detail below, the base layer 102 and/or conductive layer104 can be generated using many known techniques and materials. Incertain embodiments of the invention, the electrical circuit of thesensor is defined by etching the disposed conductive layer 104 into adesired pattern of conductive paths. A typical electrical circuit forthe sensor 100 comprises two or more adjacent conductive paths withregions at a proximal end to form contact pads and regions at a distalend to form sensor electrodes. An electrically insulating cover layer106 such as a polymer coating is optionally disposed on portions of thesensor 100. Acceptable polymer coatings for use as the insulatingprotective cover layer 106 can include, but are not limited to,non-toxic biocompatible polymers such as silicone compounds, polyimides,biocompatible solder masks, epoxy acrylate copolymers, or the like. Inthe sensors of the present invention, one or more exposed regions orapertures 108 can be made through the cover layer 106 to open theconductive layer 104 to the external environment and to, for example,allow an analyte such as glucose to permeate the layers of the sensorand be sensed by the sensing elements. Apertures 108 can be formed by anumber of techniques, including laser ablation, tape masking, chemicalmilling or etching or photolithographic development or the like. Incertain embodiments of the invention, during manufacture, a secondaryphotoresist can also be applied to the protective layer 106 to definethe regions of the protective layer to be removed to form theaperture(s) 108. The exposed electrodes and/or contact pads can alsoundergo secondary processing (e.g. through the apertures 108), such asadditional plating processing, to prepare the surfaces and/or strengthenthe conductive regions.

In the sensor configuration shown in FIG. 2, an analyte sensing layer110 (which is typically a sensor chemistry layer, meaning that materialsin this layer undergo a chemical reaction to produce a signal that canbe sensed by the conductive layer) is disposed on one or more of theexposed electrodes of the conductive layer 104. Typically, the sensorchemistry layer 110 is an enzyme layer. Most typically, the sensorchemistry layer 110 comprises an enzyme capable of producing and/orutilizing oxygen and/or hydrogen peroxide, for example the enzymeglucose oxidase. Optionally the enzyme in the sensor chemistry layer iscombined with a second carrier protein such as human serum albumin,bovine serum albumin or the like. In an illustrative embodiment, anenzyme such as glucose oxidase in the sensor chemistry layer 110 reactswith glucose to produce hydrogen peroxide, a compound which thenmodulates a current at an electrode. As this modulation of currentdepends on the concentration of hydrogen peroxide, and the concentrationof hydrogen peroxide correlates to the concentration of glucose, theconcentration of glucose can be determined by monitoring this modulationin the current. In a specific embodiment of the invention, the hydrogenperoxide is oxidized at a working electrode which is an anode (alsotermed herein the anodic working electrode), with the resulting currentbeing proportional to the hydrogen peroxide concentration. Suchmodulations in the current caused by changing hydrogen peroxideconcentrations can by monitored by any one of a variety of sensordetector apparatuses such as a universal sensor amperometric biosensordetector or one of the other variety of similar devices known in the artsuch as glucose monitoring devices produced by Medtronic MiniMed.

The analyte sensing layer 110 can be applied over portions of theconductive layer or over the entire region of the conductive layer.Typically the analyte sensing layer 110 is disposed on the workingelectrode which can be the anode or the cathode. Optionally, the analytesensing layer 110 is also disposed on a counter and/or referenceelectrode. While the analyte sensing layer 110 can be up to about 1000microns (μm) in thickness, typically the analyte sensing layer isrelatively thin as compared to those found in sensors previouslydescribed in the art, and is for example, typically less than 1, 0.5,0.25 or 0.1 microns in thickness. As discussed in detail below, somemethods for generating a thin analyte sensing layer 110 include spincoating processes, dip and dry processes, low shear spraying processes,ink-jet printing processes, silk screen processes and the like. Mosttypically the thin analyte sensing layer 110 is applied using a spincoating process.

Typically, the analyte sensing layer 110 is coated with one or moreadditional layers. Optionally, the one or more additional layersincludes a protein layer 116 disposed upon the analyte sensing layer110. Typically, the protein layer 116 comprises a protein such asalbumin or the like. Typically, the protein layer 116 comprises humanserum albumin. In some embodiments of the invention, an additional layerincludes an analyte modulating layer 112 that is disposed above theanalyte sensing layer 110 to regulate analyte contact with the analytesensing layer 110. For example, the analyte modulating membrane layer112 can comprise a glucose limiting membrane, which regulates the amountof glucose that contacts an enzyme such as glucose oxidase that ispresent in the analyte sensing layer. Such glucose limiting membranescan be made from a wide variety of materials known to be suitable forsuch purposes, e.g., silicone compounds such as polydimethyl siloxanes,polyurethanes, polyurea cellulose acetates, Nafion, polyester sulfonicacids (e.g. Kodak AQ), hydrogels or any other suitable hydrophilicmembranes known to those skilled in the art.

In typical embodiments of the invention, an adhesion promoter layer 114is disposed between the analyte modulating layer 112 and the analytesensing layer 110 as shown in FIG. 2 in order to facilitate theircontact and/or adhesion. In a specific embodiment of the invention, anadhesion promoter layer 114 is disposed between the analyte modulatinglayer 112 and the protein layer 116 as shown in FIG. 2 in order tofacilitate their contact and/or adhesion. The adhesion promoter layer114 can be made from any one of a wide variety of materials known in theart to facilitate the bonding between such layers. Typically, theadhesion promoter layer 114 comprises a silane compound. In alternativeembodiments, protein or like molecules in the analyte sensing layer 110can be sufficiently crosslinked or otherwise prepared to allow theanalyte modulating membrane layer 112 to be disposed in direct contactwith the analyte sensing layer 110 in the absence of an adhesionpromoter layer 114.

C. Typical Analyte Sensor Constitutents

The following disclosure provides examples of typicalelements/constituents used in the sensors of the invention. While theseelements can be described as discreet units (e.g. layers), those ofskill in the art understand that sensors can be designed to containelements having a combination of some or all of the material propertiesand/or functions of the elements/constituents discussed below (e.g. anelement that serves both as a supporting base constituent and/or aconductive constituent and/or a matrix for the analyte sensingconstituent and which further functions as an electrode in the sensor).

Base Constitutent

Sensors of the invention typically include a base constituent (see, e.g.element 102 in FIG. 2). The term “base constituent” is used hereinaccording to art accepted terminology and refers to the constituent inthe apparatus that typically provides a supporting matrix for theplurality of constituents that are stacked on top of one another andcomprise the functioning sensor. In one form, the base constituentcomprises a thin film sheet of insulative (e.g. electrically insulativeand/or water impermeable) material. This base constituent can be made ofa wide variety of materials having desirable qualities such as waterimpermeability and hermeticity. Some materials include metallic ceramicand polymeric substrates or the like. In certain embodiments, the baseconstituent and/or the conductive constituent can be constructed toproduce electrodes having a configuration where the electrochemicallyreactive surface area of an electrode is disposed on the geometricfeature so that the electrochemically reactive surface area is greaterthan if it was disposed on a flat surface.

The base constituent may be self-supporting or further supported byanother material as is known in the art. In one embodiment of the sensorconfiguration shown in FIG. 2, the base constituent 102 comprises aceramic. In an illustrative embodiment, the ceramic base comprises acomposition that is predominantly Al₂O₃ (e.g. 96%). The use of aluminaas an insulating base constituent for use with implantable devices isdisclosed in U.S. Pat. Nos. 4,940,858, 4,678,868 and 6,472,122 which areincorporated herein by reference. The base constituents of the inventioncan further include other elements known in the art, for examplehermetical vias (see, e.g. WO 03/023388). Depending upon the specificsensor design, the base constituent can be relatively thick constituent(e.g. thicker than 25 microns). Alternatively, one can utilize anonconductive ceramic, such as alumina, in thin constituents, e.g., lessthan about 25 microns.

Embodiments of invention disclosed herein provide individual elementsand sensors which exhibit a combination of the independent advantagesfound in each of the two sensor classes disclosed above. For example afirst embodiment of the invention immobilizes an enzyme onto a thick(1-1,000 micron), porous substrate which functions as an electrode inthe sensor. In this context, the porous electrode is designed to exhibitan increased surface area, for example by constructing it from a latticeof equal-sized adjoining spheres. In one illustrative embodiment,glucose oxidase is immobilized on a thick (1-1,000 micron), porousmetallic substrate that is manufactured from a lattice of equal-sizedadjoining spheres and which function as a hydrogen peroxide-consumingelectrode.

The advantage of such a thick, porous electrode matrices relative tothin, flat electrode matrices is demonstrated using Equation (1):

$\begin{matrix}{\frac{A_{avail}}{A_{proj}} = \frac{3{L\left( {{1 -} \in} \right)}\varphi}{R}} & (1)\end{matrix}$

where the thick, porous electrode is modeled as a lattice of equal-sizedadjoining spheres, while the thin electrode is modeled as atwo-dimensional surface. The surface area available for enzyme orprotein immobilization is A_(avail), while the projected area of theelectrode is A_(proj). The porosity and thickness of the electrode are Land ε, respectively. The spheres making-up the thick electrode are ofradius R, while the fraction of the spheres' surface area available forenzyme or protein immobilization is φ. For example, a porous electrodewith L=25 μm, R=1 μm, ε=0.5, and φ=0.5 would possess more than 18 timesthe surface area for enzyme immobilization as compared to a thinelectrode with same projected area.

The porosity range of the such as the porous electrode matricesdiscussed above is typically 5-99%, 10-99%, 20-99%, 30-99%, 40-99%,50-99% or 60-99%. The porosity of matrices can be evaluated using anyone of a variety of methods known in the art. In certain contexts forexample, artisans may wish to examine porosity of a matrix via mercuryporosimetry (see, e.g. U.S. Pat. No. 5,609,839), liquid intrusionporosimetry (see, e.g. U.S. Pat. No. 4,660,412), gas porosimetry (see,e.g. Dombrowski et al., Langmuir 16: 5041-5050 (2000) and Lastoskie etal., Journal of Physical Chemistry 97: 4786-4796 (1993)), or by cyclicvoltametry and/or methods which employ size exclusion chromatographyusing marker molecules of various sizes and molecular weights (e.g.acetone, various globular proteins, blue dextran etc.).

The terms nano-porous, micro-porous and macro-porous are used whendiscussing certain embodiments of the porous matrices that are disclosedherein. For example, platinum-black is commonly used to increase theelectrochemically effective surface area of a working electrode.Standard platinum-black electrodes have a great deal of porosity, withthe pores being sized so that only very small molecules like H₂, O₂, andH₂O can get inside them. Platinum-black electrodes having thischaracteristic are termed nano-porous. Such nano-pores have a size rangethat permits small molecules like H₂, O₂, and H₂O the get inside them,but prevents larger molecules like GOx from getting inside. In certainembodiments of the invention, the electrodes used in the sensors haveboth nano- and micro-porosity. Micro-pores are characterized in thatthey are large enough to allow molecules such as GOx to be immobilizedinside of them, but are small enough so that any molecule of GOx isrelatively close (less than about 0.1, 1, or 2 microns) to the surfaceof the working electrode. Electrodes having this micro-porosity exhibita number of desirable characteristics. For example, as the workingelectrode of an H₂O₂-based sensor consumes H₂O₂ and H₂O₂ is believed tocontribute to the deactivation GOx over time, micro-porous electrodesthat allow the placement of immobilized GOx in close proximity to anH₂O₂-consuming electrode will increase the lifetime of GOx in thesensor.

In another embodiment of the invention disclosed herein the hydrogeltypically utilized in a variety of analyte sensors is replaced with anessentially rigid, non-swelling porous enzyme-polymer matrix. In thisembodiment, bio-sensing enzymes can be stably immobilized via covalentbonding to a rigid, macroporous polymer that has optionally been moldedinto a specified shape. In this context, molded continuous rods ofmacroporous polymers have been developed for use as chromatographicseparation media (see, e.g. U.S. Pat. No. 5,453,185 and WO 93/07945).Suitable polymers are essentially incompressible and do not change theiroverall size in response to changes in their solvating environment.Moreover, adjustments to the polymerization conditions can be used tocontrol the morphology of the pores. Hence, highly porous (50-70%)polymers can be created that possess significant volume fractions ofpores in the ranges of 1-100 nm and 100-3,000 nm (i.e. 20% and 80%,respectively). Polymers with this type of pore structure possess a veryhigh specific surface area (i.e. 185 m²/g), and are expected to allowfor high enzyme immobilization densities (1-100 mg/mL).

Various methods and compositions for making and using the above-notedporous matrices as well as analyte sensors which incorporate suchmatrices are further described herein.

Conductive Constitutent

The electrochemical sensors of the invention typically include aconductive constituent disposed upon the base constituent that includesat least one electrode for contacting an analyte or its byproduct (e.g.oxygen and/or hydrogen peroxide) to be assayed (see, e.g. element 104 inFIG. 2). The term “conductive constituent” is used herein according toart accepted terminology and refers to electrically conductive sensorelements such as electrodes which are capable of measuring and adetectable signal and conducting this to a detection apparatus. Anillustrative example of this is a conductive constituent that canmeasure an increase or decrease in current in response to exposure to astimuli such as the change in the concentration of an analyte or itsbyproduct as compared to a reference electrode that does not experiencethe change in the concentration of the analyte, a coreactant (e.g.oxygen) used when the analyte interacts with a composition (e.g. theenzyme glucose oxidase) present in analyte sensing constituent 110 or areaction product of this interaction (e.g. hydrogen peroxide).Illustrative examples of such elements include electrodes which arecapable of producing a variable detectable signals in the presence ofvariable concentrations of molecules such as hydrogen peroxide oroxygen. Typically one of these electrodes in the conductive constituentis a working electrode, which can be made from non-corroding metal orcarbon. A carbon working electrode may be vitreous or graphitic and canbe made from a solid or a paste. A metallic working electrode may bemade from platinum group metals, including palladium or gold, or anon-corroding metallically conducting oxide, such as ruthenium dioxide.Alternatively the electrode may comprise a silver/silver chlorideelectrode composition. The working electrode may be a wire or a thinconducting film applied to a substrate, for example, by coating orprinting. Typically, only a portion of the surface of the metallic orcarbon conductor is in electrolytic contact with the analyte-containingsolution. This portion is called the working surface of the electrode.The remaining surface of the electrode is typically isolated from thesolution by an electrically insulating cover constituent 106. Examplesof useful materials for generating this protective cover constituent 106include polymers such as polyimides, polytetrafluoroethylene,polyhexafluoropropylene and silicones such as polysiloxanes.

In addition to the working electrode, the analyte sensors of theinvention typically include a reference electrode or a combinedreference and counter electrode (also termed a quasi-reference electrodeor a counter/reference electrode). If the sensor does not have acounter/reference electrode then it may include a separate counterelectrode, which may be made from the same or different materials as theworking electrode. Typical sensors of the present invention have one ormore working electrodes and one or more counter, reference, and/orcounter/reference electrodes. One embodiment of the sensor of thepresent invention has two, three or four or more working electrodes.These working electrodes in the sensor may be integrally connected orthey may be kept separate.

Typically, for in vivo use the analyte sensors of the present inventionare implanted subcutaneously in the skin of a mammal for direct contactwith the body fluids of the mammal, such as blood. Alternatively thesensors can be implanted into other regions within the body of a mammalsuch as in the intraperotineal space. When multiple working electrodesare used, they may be implanted together or at different positions inthe body. The counter, reference, and/or counter/reference electrodesmay also be implanted either proximate to the working electrode(s) or atother positions within the body of the mammal.

Interference Rejection Constitutent

The electrochemical sensors of the invention optionally include aninterference rejection constituent disposed between the surface of theelectrode and the environment to be assayed. In particular, certainsensor embodiments rely on the oxidation and/or reduction of hydrogenperoxide generated by enzymatic reactions on the surface of a workingelectrode at a constant potential applied. Because amperometricdetection based on direct oxidation of hydrogen peroxide requires arelatively high oxidation potential, sensors employing this detectionscheme may suffer interference from oxidizable species that are presentin biological fluids such as ascorbic acid, uric acid and acetaminophen.In this context, the term “interference rejection constituent” is usedherein according to art accepted terminology and refers to a coating ormembrane in the sensor that functions to inhibit spurious signalsgenerated by such oxidizable species which interfere with the detectionof the signal generated by the analyte to be sensed. Examples ofinterference rejection constituents include one or more layers orcoatings of compounds such as hydrophilic polyurethanes, celluloseacetate (including cellulose acetate incorporating agents such aspoly(ethylene glycol), polyethersulfones, polytetra-fluoroethylenes, theperfluoronated ionomer Naflon™, polyphenylenediamine, epoxy and thelike. Illustrative discussions of such interference rejectionconstituents are found for example in Ward et al., Biosensors andBioelectronics 17 (2002) 181-189 and Choi et al., Analytical ChimicaActa 461 (2002) 251-260 which are incorporated herein by reference.

Analyte Sensing Constitutent

The electrochemical sensors of the invention include an analyte sensingconstituent disposed on the electrodes of the sensor (see, e.g. element110 in FIG. 2). The term “analyte sensing constituent” is used hereinaccording to art accepted terminology and refers to a constituentcomprising a material that is capable of recognizing or reacting with ananalyte whose presence is to be detected by the analyte sensorapparatus. Typically this material in the analyte sensing constituentproduces a detectable signal after interacting with the analyte to besensed, typically via the electrodes of the conductive constituent. Inthis regard the analyte sensing constituent and the electrodes of theconductive constituent work in combination to produce the electricalsignal that is read by an apparatus associated with the analyte sensor.Typically, the analyte sensing constituent comprises an enzyme capableof reacting with and/or producing a molecule whose change inconcentration can be measured by measuring the change in the current atan electrode of the conductive constituent (e.g. oxygen and/or hydrogenperoxide), for example the enzyme glucose oxidase. An enzyme capable ofproducing a molecule such as hydrogen peroxide can be disposed on theelectrodes according to a number of processes known in the art. Theanalyte sensing constituent can coat all or a portion of the variouselectrodes of the sensor. In this context, the analyte sensingconstituent may coat the electrodes to an equivalent degree.Alternatively the analyte sensing constituent may coat differentelectrodes to different degrees, with for example the coated surface ofthe working electrode being larger than the coated surface of thecounter and/or reference electrode.

Typical sensor embodiments of this element of the invention utilize anenzyme (e.g. glucose oxidase) that has been combined with a secondprotein (e.g. albumin) in a fixed ratio (e.g. one that is typicallyoptimized for glucose oxidase stabilizing properties) and then appliedon the surface of an electrode to form a thin enzyme constituent. In atypical embodiment, the analyte sensing constituent comprises a GOx andHSA mixture. A typical embodiment of an analyte sensing constituenthaving GOx, the GOx reacts with glucose present in the sensingenvironment (e.g. the body of a mammal) and generates hydrogen peroxideaccording to the reaction shown in FIG. 1, wherein the hydrogen peroxideso generated is anodically detected at the working electrode in theconductive constituent. As discussed for example in U.S. patentapplication Ser. No. 10/273,767 (incorporated herein by reference)extremely thin sensor chemistry constituents are typical and can beapplied to the surface of the electrode matrix by processes known in theart such as spin coating. In an illustrative embodiment, a glucoseoxidase/albumin is prepared in a physiological solution (e.g., phosphatebuffered saline at neutral pH) with the albumin being present in anrange of about 0.5%-10% by weight. Optionally the stabilized glucoseoxidase constituent that is formed on the analyte sensing constituent isvery thin as compared to those previously described in the art, forexample less than 2, 1, 0.5, 0.25 or 0.1 microns in thickness. Oneillustrative embodiment of the invention utilizes a stabilized glucoseoxidase constituent for coating the surface of an electrode wherein theglucose oxidase is mixed with a carrier protein in a fixed ratio withinthe constituent, and the glucose oxidase and the carrier protein aredistributed in a substantially uniform manner throughout theconstituent. Typically the constituent is less than 2 microns inthickness. For purposes of clarity, it should be noted that this may notapply to certain embodiments of the invention where the analyte sensingconstituent is disposed on a porous electrode. For example, in a porouselectrode that is 100 microns thick, with 3 micron size pores that arefilled with Gox, an enzyme layer can be greater 2 microns.

Surprisingly, sensors having these extremely thin analyte sensingconstituents have material properties that exceed those of sensorshaving thicker coatings including enhanced longevity, linearity,regularity as well as improved signal to noise ratios. While not beingbound by a specific scientific theory, it is believed that sensorshaving extremely thin analyte sensing constituents have surprisinglyenhanced characteristics as compared to those of thicker constituentsbecause in thicker enzyme constituents only a fraction of the reactiveenzyme within the constituent is able to access the analyte to besensed. In sensors utilizing glucose oxidase, the thick coatingsproduced by electrodeposition may hinder the ability of hydrogenperoxide generated at the reactive interface of a thick enzymeconstituent to contact the sensor surface and thereby generate a signal.

As noted above, the enzyme and the second protein are typically treatedto form a crosslinked matrix (e.g. by adding a cross-linking agent tothe protein mixture). As is known in the art, crosslinking conditionsmay be manipulated to modulate factors such as the retained biologicalactivity of the enzyme, its mechanical and/or operational stability.Illustrative crosslinking procedures are described in U.S. patentapplication Ser. No. 10/335,506 and PCT publication WO 03/035891 whichare incorporated herein by reference. For example, an aminecross-linking reagent, such as, but not limited to, glutaraldehyde, canbe added to the protein mixture. The addition of a cross-linking reagentto the protein mixture creates a protein paste. The concentration of thecross-linking reagent to be added may vary according to theconcentration of the protein mixture. While glutaraldehyde is anillustrative crosslinking reagent, other cross-linking reagents may alsobe used or may be used in place of glutaraldehyde, including, but notlimited to, an amine reactive, homofunctional, cross-linking reagentsuch as Disuccinimidyl Suberate (DSS). Another example is 1-Ethyl-3(3-Dimethylaminopropyl) Carbodiimide (EDC), which is a zero-lengthcross-linker. EDC forms an amide bond between carboxylic acid and aminegroups. Other suitable cross-linkers also may be used, as will beevident to those skilled in the art.

The GOx and/or carrier protein concentration may vary for differentembodiments of the invention. For example, the GOx concentration may bewithin the range of approximately 50 mg/ml (approximately 10,000 U/ml)to approximately 700 mg/ml (approximately 150,000 U/ml). Typically theGOx concentration is about 115 mg/ml (approximately 22,000 U/ml). Insuch embodiments, the HSA concentration may vary between about 0.5%-30%(w/v), depending on the GOx concentration. Typically the HSAconcentration is about 1-10% w/v, and most typically is about 5% w/v. Inalternative embodiments of the invention, collagen or BSA or otherstructural proteins used in these contexts can be used instead of or inaddition to HSA. Although GOx is discussed as an illustrative enzyme inthe analyte sensing constituent, other proteins and/or enzymes may alsobe used or may be used in place of GOx, including, but not limited toglucose dehydrogenase or hexokinase, hexose oxidase, lactate oxidase,and the like. Other proteins and/or enzymes may also be used, as will beevident to those skilled in the art. Moreover, although HSA is employedin the example embodiment, other structural proteins, such as BSA,collagens or the like, could be used instead of or in addition to HSA.

For embodiments employing enzymes other than GOx, concentrations otherthan those discussed herein may be utilized. For example, depending onthe enzyme employed, concentrations ranging from approximately 10%weight per weight to 70% weight per weight may be suitable. Theconcentration may be varied not only depending on the particular enzymebeing employed, but also depending on the desired properties of theresulting protein matrix. For example, a certain concentration may beutilized if the protein matrix is to be used in a diagnostic capacitywhile a different concentration may be utilized if certain structuralproperties are desired. Those skilled in the art will understand thatthe concentration utilized may be varied through experimentation todetermine which concentration (and of which enzyme or protein) may yieldthe desired result.

As noted above, in some embodiments of the invention, the analytesensing constituent includes a composition (e.g. glucose oxidase)capable of producing a signal (e.g. a change in oxygen and/or hydrogenperoxide concentrations) that can be sensed by the electricallyconductive elements (e.g. electrodes which sense changes in oxygenand/or hydrogen peroxide concentrations). However, other useful analytesensing constituents can be formed from any composition that is capableof producing a detectable signal that can be sensed by the electricallyconductive elements after interacting with a target analyte whosepresence is to be detected. In some embodiments, the compositioncomprises an enzyme that modulates hydrogen peroxide concentrations uponreaction with an analyte to be sensed. Alternatively, the compositioncomprises an enzyme that modulates oxygen concentrations upon reactionwith an analyte to be sensed. In this context, a wide variety of enzymesthat either use or produce hydrogen peroxide and/or oxygen in a reactionwith a physiological analyte are known in the art and these enzymes canbe readily incorporated into the analyte sensing constituentcomposition. A variety of other enzymes known in the art can produceand/or utilize compounds whose modulation can be detected byelectrically conductive elements such as the electrodes that areincorporated into the sensor designs described herein. Such enzymesinclude for example, enzymes specifically described in Table 1, pages15-29 and/or Table 18, pages 111-112 of Protein Immobilization:Fundamentals and Applications (Bioprocess Technology, Vol 14) by RichardF. Taylor (Editor) Publisher: Marcel Dekker; (Jan. 7, 1991) the entirecontents of which are incorporated herein by reference.

Other useful analyte sensing constituents can be formed to includeantibodies whose interaction with a target analyte is capable ofproducing a detectable signal that can be sensed by the electricallyconductive elements after interacting with the target analyte whosepresence is to be detected. For example U.S. Pat. No. 5,427,912 (whichis incorporated herein by reference) describes an antibody-basedapparatus for electrochemically determining the concentration of ananalyte in a sample. In this device, a mixture is formed which includesthe sample to be tested, an enzyme-acceptor polypeptide, an enzyme-donorpolypeptide linked to an analyte analog (enzyme-donor polypeptideconjugate), a labeled substrate, and an antibody specific for theanalyte to be measured. The analyte and the enzyme-donor polypeptideconjugate competitively bind to the antibody. When the enzyme-donorpolypeptide conjugate is not bound to antibody, it will spontaneouslycombine with the enzyme acceptor polypeptide to form an active enzymecomplex. The active enzyme then hydrolyzes the labeled substrate,resulting in the generation of an electroactive label, which can then beoxidized at the surface of an electrode. A current resulting from theoxidation of the electroactive compound can be measured and correlatedto the concentration of the analyte in the sample. U.S. Pat. No.5,149,630 (which is incorporated herein by reference) describes anelectrochemical specific binding assay of a ligand (e.g., antigen,hapten or antibody) wherein at least one of the components isenzyme-labelled, and which includes the step of determining the extentto which the transfer of electrons between the enzyme substrate and anelectrode, associated with the substrate reaction, is perturbed bycomplex formation or by displacement of any ligand complex relative tounbound enzyme-labelled component. The electron transfer is aided byelectron-transfer mediators which can accept electrons from the enzymeand donate them to the electrode or vice versa (e.g. ferrocene) or byelectron-transfer promoters which retain the enzyme in close proximitywith the electrode without themselves taking up a formal charge. U.S.Pat. No. 5,147,781 (which is incorporated herein by reference) describesan assay for the determination of the enzyme lactate dehydrogenase-5(LDH5) and to a biosensor for such quantitative determination. The assayis based on the interaction of this enzyme with the substrate lacticacid and nicotine-amine adenine dinucleotide (NAD) to yield pyruvic acidand the reduction product of NAD. Anti-LDH5 antibody is bound to asuitable glassy carbon electrode; this is contacted with the substratecontaining LDH5, rinsed, inserted into a NAD solution, connected to anamperometric system, current changes are measured in the presence ofdiffering concentrations of lactic acid, which are indicative of thequantity of LDH-5. U.S. Pat. No. 6,410,251 (which is incorporated hereinby reference) describes an apparatus and method for detecting orassaying one constituting member in a specific binding pair; forexample, the antigen in an antigen/antibody pair, by utilizing specificbinding such as binding between an antigen and an antibody, togetherwith redox reaction for detecting a label, wherein an oxygenmicro-electrode with a sensing surface area is used. In addition, U.S.Pat. No. 4,402,819 (which is incorporated herein by reference) describesan antibody-selective potentiometric electrode for the quantitativedetermination of antibodies (as the analyte) in dilute liquid serumsamples employing an insoluble membrane incorporating an antigen havingbonded thereto an ion carrier effecting the permeability of preselectedcations therein, which permeability is a function of specific antibodyconcentrations in analysis, and the corresponding method of analysis.For related disclosures, see also U.S. Pat. Nos. 6,703,210, 5,981,203,5,705,399 and 4,894,253, the contents of which are incorporated hereinby reference.

In addition to enzymes and antibodies, other exemplary materials for usein the analyte sensing constituents of the sensors disclosed hereininclude polymers that bind specific types of cells or cell components(e.g. polypeptides, carbohydrates and the like); single-strand DNA;antigens and the like. The detectable signal can be, for example, anoptically detectable change, such as a color change or a visibleaccumulation of the desired analyte (e.g., cells). Sensing elements canalso be formed from materials that are essentially non-reactive (i.e.,controls). The foregoing alternative sensor elements are beneficiallyincluded, for example, in sensors for use in cell-sorting assays andassays for the presence of pathogenic organisms, such as viruses (HIV,hepatitis-C, etc.), bacteria, protozoa and the like.

Also contemplated are analyte sensors that measure an analyte that ispresent in the external environment and that can in itself produce ameasurable change in current at an electrode. In sensors measuring suchanalytes, the analyte sensing constituent can be optional.

Protein Constitutent

The electrochemical sensors of the invention optionally include aprotein constituent disposed between the analyte sensing constituent andthe analyte modulating constituent (see, e.g. element 116 in FIG. 2).The term “protein constituent” is used herein according to art acceptedterminology and refers to constituent containing a carrier protein orthe like that is selected for compatibility with the analyte sensingconstituent and/or the analyte modulating constituent. In typicalembodiments, the protein constituent comprises an albumin such as humanserum albumin. The HSA concentration may vary between about 0.5%-30%(w/v). Typically the HSA concentration is about 1-10% w/v, and mosttypically is about 5% w/v. In alternative embodiments of the invention,collagen or BSA or other structural proteins used in these contexts canbe used instead of or in addition to HSA. This constituent is typicallycrosslinked on the analyte sensing constituent according to art acceptedprotocols.

Adhesion Promoting Constitutent

The electrochemical sensors of the invention can include one or moreadhesion promoting (AP) constituents (see, e.g. element 114 in FIG. 2).The term “adhesion promoting constituent” is used herein according toart accepted terminology and refers to a constituent that includesmaterials selected for their ability to promote adhesion betweenadjoining constituents in the sensor. Typically, the adhesion promotingconstituent is disposed between the analyte sensing constituent and theanalyte modulating constituent. Typically, the adhesion promotingconstituent is disposed between the optional protein constituent and theanalyte modulating constituent. The adhesion promoter constituent can bemade from any one of a wide variety of materials known in the art tofacilitate the bonding between such constituents and can be applied byany one of a wide variety of methods known in the art. Typically, theadhesion promoter constituent comprises a silane compound such asγ-aminopropyltrimethoxysilane.

The use of silane coupling reagents, especially those of the formulaR′Si(OR)₃ in which R′ is typically an aliphatic group with a terminalamine and R is a lower alkyl group, to promote adhesion is known in theart (see, e.g. U.S. Pat. No. 5,212,050 which is incorporated herein byreference). For example, chemically modified electrodes in which asilane such as γ-aminopropyltriethoxysilane and glutaraldehyde were usedin a step-wise process to attach and to co-crosslink bovine serumalbumin (BSA) and glucose oxidase (GOx) to the electrode surface arewell known in the art (see, e.g. Yao, T. Analytica Chim. Acta 1983, 148,27-33).

In certain embodiments of the invention, the adhesion promotingconstituent further comprises one or more compounds that can also bepresent in an adjacent constituent such as the polydimethyl siloxane(PDMS) compounds that serves to limit the diffusion of analytes such asglucose through the analyte modulating constituent. In illustrativeembodiments the formulation comprises 0.5-20% PDMS, typically 5-15%PDMS, and most typically 10% PDMS. In certain embodiments of theinvention, the adhesion promoting constituent includes an agent selectedfor its ability to crosslink a siloxane moiety present in a proximalconstituent such as the analyte modulating constituent. In closelyrelated embodiments of the invention, the adhesion promoting constituentincludes an agent selected for its ability to crosslink an amine orcarboxyl moiety of a protein present in a proximal constituent such athe analyte sensing constituent and/or the protein constituent.

Analyte Modulating Constitutent

The electrochemical sensors of the invention include an analytemodulating constituent disposed on the sensor (see, e.g. element 112 inFIG. 2). The term “analyte modulating constituent” is used hereinaccording to art accepted terminology and refers to a constituent thattypically forms a membrane on the sensor that operates to modulate thediffusion of one or more analytes, such as glucose, through theconstituent. In certain embodiments of the invention, the analytemodulating constituent is an analyte-limiting membrane which operates toprevent or restrict the diffusion of one or more analytes, such asglucose, through the constituents. In other embodiments of theinvention, the analyte-modulating constituent operates to facilitate thediffusion of one or more analytes, through the constituents. Optionallysuch analyte modulating constituents can be formed to prevent orrestrict the diffusion of one type of molecule through the constituent(e.g. glucose), while at the same time allowing or even facilitating thediffusion of other types of molecules through the constituent (e.g. O₂).

With respect to glucose sensors, in known enzyme electrodes, glucose andoxygen from blood, as well as some interferants, such as ascorbic acidand uric acid, diffuse through a primary membrane of the sensor. As theglucose, oxygen and interferants reach the analyte sensing constituent,an enzyme, such as glucose oxidase, catalyzes the conversion of glucoseto hydrogen peroxide and gluconolactone. The hydrogen peroxide maydiffuse back through the analyte modulating constituent, or it maydiffuse to an electrode where it can be reacted to form oxygen and aproton to produce a current that is proportional to the glucoseconcentration. The sensor membrane assembly serves several functions,including selectively allowing the passage of glucose therethrough. Inthis context, an illustrative analyte modulating constituent is asemi-permeable membrane which permits passage of water, oxygen and atleast one selective analyte and which has the ability to absorb water,the membrane having a water soluble, hydrophilic polymer.

A variety of illustrative analyte modulating compositions are known inthe art and are described for example in U.S. Pat. Nos. 6,319,540,5,882,494, 5,786,439 5,777,060, 5,771,868 and 5,391,250, the disclosuresof each being incorporated herein by reference. The hydrogels describedtherein are particularly useful with a variety of implantable devicesfor which it is advantageous to provide a surrounding water constituent.In some embodiments of the invention, the analyte modulating compositionincludes PDMS. In certain embodiments of the invention, the analytemodulating constituent includes an agent selected for its ability tocrosslink a siloxane moiety present in a proximal constituent. Inclosely related embodiments of the invention, the adhesion promotingconstituent includes an agent selected for its ability to crosslink anamine or carboxyl moiety of a protein present in a proximal constituent.

Cover Constitutent

The electrochemical sensors of the invention include one or more coverconstituents which are typically electrically insulating protectiveconstituents (see, e.g. element 106 in FIG. 2). Typically, such coverconstituents are disposed on at least a portion of the analytemodulating constituent. Acceptable polymer coatings for use as theinsulating protective cover constituent can include, but are not limitedto, non-toxic biocompatible polymers such as silicone compounds,polyimides, biocompatible solder masks, epoxy acrylate copolymers, orthe like. Further, these coatings can be photo-imageable to facilitatephotolithographic forming of apertures through to the conductiveconstituent. A typical cover constituent comprises spun on silicone. Asis known in the art, this constituent can be a commercially availableRTV (room temperature vulcanized) silicone composition. A typicalchemistry in this context is polydimethyl siloxane (acetoxy based).

Various illustrative embodiments of the invention and theircharacteristics are discussed in detail in the following sections.

D. Illustrative Embodiments of Analyte Sensor Apparatus and AssociatedCharacteristics

The analyte sensor apparatus disclosed herein has a number ofembodiments. A general embodiment of the invention is an analyte sensorapparatus for implantation within a mammal. While the analyte sensorsare typically designed to be implantable within the body of a mammal,the sensors are not limited to any particular environment can instead beused in a wide variety of contexts, for example for the analysis of mostliquid samples including biological fluids such as whole-blood, lymph,plasma, serum, saliva, urine, stool, perspiration, mucus, tears,cerebrospinal fluid, nasal secretion, cervical or vaginal secretion,semen, pleural fluid, amniotic fluid, peritoneal fluid, middle earfluid, joint fluid, gastric aspirate or the like. In addition, solid ordesiccated samples may be dissolved in an appropriate solvent to providea liquid mixture suitable for analysis.

As noted above, the sensor embodiments disclosed herein can be used tosense analytes of interest in one or more physiological environments. Incertain embodiments for example, the sensor can be in direct contactwith interstitial fluids as typically occurs with subcutaneous sensors.The sensors of the present invention may also be part of a skin surfacesystem where interstitial glucose is extracted through the skin andbrought into contact with the sensor (see, e.g. 6,155,992 and 6,706,159which are incorporated herein by reference). In other embodiments, thesensor can be in contact with blood as typically occurs for example withintravenous sensors. The sensor embodiments of the invention furtherinclude those adapted for use in a variety of contexts. In certainembodiments for example, the sensor can be designed for use in mobilecontexts, such as those employed by ambulatory users. Alternatively, thesensor can be designed for use in stationary contexts such as thoseadapted for use in clinical settings. Such sensor embodiments include,for example, those used to monitor one or more analytes present in oneor more physiological environments in a hospitalized patient.

Sensors of the invention can also be incorporated in to a wide varietyof medical systems known in the art. Sensors of the invention can beused, for example, in a closed loop infusion systems designed to controlthe rate that medication is infused into the body of a user. Such aclosed loop infusion system can include a sensor and an associated meterwhich generates an input to a controller which in turn operates adelivery system (e.g. one that calculates a dose to be delivered by amedication infusion pump). In such contexts, the meter associated withthe sensor may also transmit commands to, and be used to remotelycontrol, the delivery system. Typically, the sensor is a subcutaneoussensor in contact with interstitial fluid to monitor the glucoseconcentration in the body of the user, and the liquid infused by thedelivery system into the body of the user includes insulin. Illustrativesystems are disclosed for example in U.S. Pat. Nos. 6,558,351 and6,551,276; PCT Application Nos. US99/21703 and US99/22993; as well as WO2004/008956 and WO 2004/009161, all of which are incorporated herein byreference.

Certain embodiments of the invention measure peroxide and have theadvantageous characteristic of being suited for implantation in avariety of sites in the mammal including regions of subcutaneousimplantation and intravenous implantation as well as implantation into avariety of non-vascular regions. A peroxide sensor design that allowsimplantation into non-vascular regions has advantages over certainsensor apparatus designs that measure oxygen due to the problems withoxygen noise that can occur in oxygen sensors implanted intonon-vascular regions. For example, in such implanted oxygen sensorapparatus designs, oxygen noise at the reference sensor can compromisethe signal to noise ratio which consequently perturbs their ability toobtain stable glucose readings in this environment. The peroxide sensorsof the invention therefore overcome the difficulties observed with suchoxygen sensors in non-vascular regions.

Certain peroxide sensor embodiments of the invention further includeadvantageous long term or “permanent” sensors which are suitable forimplantation in a mammal for a time period of greater than 30 days. Inparticular, as is known in the art (see, e.g. ISO 10993, BiologicalEvaluation of Medical Devices) medical devices such as the sensorsdescribed herein can be categorized into three groups based on implantduration: (1) “Limited” (<24 hours), (2) “Prolonged” (24 hours-30 days),and (3) “Permanent” (>30 days). In some embodiments of the invention,the design of the peroxide sensor of the invention allows for a“Permanent” implantation according to this categorization, i.e. >30days. In related embodiments of the invention, the highly stable designof the peroxide sensor of the invention allows for an implanted sensorto continue to function in this regard for 2, 3, 4, 5, 6 or 12 or moremonths.

In general, the analyte sensor apparatus structure comprises a baselayer and a conductive layer disposed upon the base layer (e.g. a porousmatrix) and functions as one or more electrodes. For example, theconductive layer can include a working electrode, a reference electrodeand/or a counter electrode. These electrodes can be spaced in proximity,or alternatively are spaced distally, according to the specific design.The sensor apparatus design is such that certain electrodes (e.g. theworking electrode) can be exposed to the solution containing the analyteto be sensed (e.g. via an aperture) in the sensor apparatus. The sensorapparatus design is such that certain electrodes (e.g. the referenceelectrode) are not exposed to the solution containing the analyte to besensed in the sensor apparatus.

One embodiment of the invention is a composition for use in biosensors.Such compositions are typically designed to implantable within a mammaland comprise a porous matrix having a surface coated with an immobilizedenzyme, for example glucose oxidase, glucose dehydrogenase, lactateoxidase, hexokinase or lactate dehydrogenase. Typically the porousmatrix coated with an immobilized enzyme is capable of acting as anelectrode in an electrochemical sensor. Optionally the electrode in theelectrochemical sensor consumes hydrogen peroxide.

The porous matrices used in various embodiments of the biosensors of theinvention can be generated from a variety of materials and can beadapted to a variety of compositional configurations. In someembodiments of the invention, the porous matrix comprises a ceramicmaterial and/or a metal and/or a macroporous polymer. Optionally theporous matrix comprises a lattice of particles. Typically the particlesare spherical. In typical embodiments of the invention, porous matrixhas a surface area that is at least 2, 4, 6, 8, 10, 12, 14, 16 or 18times the surface area of a non-porous matrix of same dimensions. Incertain embodiments of the invention, the porous matrix is at least 1,10, 100, or 1000 microns thick. In certain embodiments of the invention,the porosity range of the porous matrix is optionally about 5-99.9% andtypically is about 40-99%. The porosity of these matrices can bemeasured by one of the protocols typically used in the art such asmercury or gas porosimetry, size-exclusion chromatography using markermolecules of various sizes and molecular weights (e.g. acetone, variousglobular proteins of a defined size, blue dextran), and cyclicvoltammetry.

A related embodiment of the invention is an analyte sensor apparatus forimplantation within a mammal which includes a porous matrix having asurface coated with an immobilized enzyme, for example glucose oxidase.In one embodiment of this sensor design, the porous matrix comprises aworking electrode; and the immobilized enzyme is disposed within ananalyte sensing layer disposed on the working electrode, such that theanalyte sensing layer detectably alters the electrical current at theworking electrode in the conductive layer in the presence of an analyte.Typically the sensor further comprises an analyte modulating layerdisposed on the analyte sensing layer, wherein the analyte modulatinglayer modulates the diffusion of the analyte therethrough. Typically,the sensor further comprises an adhesion promoting layer disposed on theanalyte sensing layer, wherein the adhesion promoting layer promotes theadhesion between the analyte sensing layer and an analyte modulatinglayer disposed on the analyte sensing layer. Optionally the sensorfurther comprises a protein layer disposed between the analyte sensinglayer and the analyte modulating layer. Typically the sensor furthercomprises a cover layer disposed on at least a portion of the analytemodulating layer, wherein the cover layer further includes an aperturethat exposes at least a portion of the analyte modulating layer to asolution comprising the analyte to be sensed.

A related embodiment of the invention is a method of making a sensorapparatus for implantation within a mammal comprising the steps ofproviding a layer comprising a porous matrix, forming an analyte sensinglayer on the porous matrix, wherein the analyte sensing layer includesan enzyme such as glucose oxidase that can alter the electrical currentat the surface of the porous matrix in the presence of an analyte sothat the porous matrix having the analyte sensing layer formed thereonfunctions as an electrode. Such methods further include the steps ofoptionally forming a protein layer on the analyte sensing layer, formingan adhesion promoting layer on the analyte sensing layer or the optionalprotein layer, forming an analyte modulating layer disposed on theadhesion promoting layer, wherein the analyte modulating layer includesa composition that modulates the diffusion of the analyte therethrough;and forming a cover layer disposed on at least a portion of the analytemodulating layer, wherein the cover layer further includes an apertureover at least a portion of the analyte modulating layer.

Another embodiment of the invention is a method of sensing an analytewithin the body of a mammal, the method comprising implanting an analytesensor into the mammal, the analyte sensor comprising a porous matrixhaving an analyte sensing layer disposed thereon, wherein the analytesensing layer detectably alters the electrical current at the surface ofthe porous matrix in the presence of an analyte so that the porousmatrix having the analyte sensing layer formed thereon functions as anelectrode, an optional protein layer disposed on the analyte sensinglayer, an adhesion promoting layer disposed on the analyte sensing layeror the optional protein layer, wherein the adhesion promoting layerpromotes the adhesion between the analyte sensing layer and an analytemodulating layer disposed on the analyte sensing layer, and an analytemodulating layer disposed on the analyte sensing layer, wherein theanalyte modulating layer modulates the diffusion of the analytetherethrough, a cover layer disposed on at least a portion of theanalyte modulating layer, wherein the cover layer further includes anaperture over at least a portion of the analyte modulating layer; andsensing an alteration in electrical current and correlating thealteration in current with the presence of the analyte, so that theanalyte is sensed.

Yet another embodiment of the invention is a method of immobilizing aprotein on a rigid macroporous polymer comprising the steps of:combining the protein with the rigid macroporous polymer havingfunctional moieties capable of crosslinking to a protein; and thenadding a crosslinking agent capable of immobilizing the protein on therigid macroporous polymer by crosslinking the functional moieties of theprotein with the functional moieties of the rigid macroporous polymer sothat the protein is immobilized on the rigid macroporous polymer. Incertain embodiments of the invention, the rigid macroporous polymerhaving functional moieties capable of crosslinking to a protein is madeby combining a rigid macroporous polymer having reactive epoxidemoieties with a nucleophilic compound so that a rigid macroporouspolymer having functional moieties capable of crosslinking to a proteinis made.

Yet another embodiment of the invention is a method of immobilizing aprotein on a rigid macroporous polymer comprising combining a proteinhaving a sulfhydryl, amine, carboxyl or hydroxyl moiety with a rigidmacroporous polymer having reactive epoxide moieties under reactionconditions that allow a nucleophilic reaction to occur between thesulfhydryl, amine, carboxyl or hydroxyl moieties on the protein and theepoxide moieties on the rigid macroporous polymer so that the protein isimmobilized on the rigid macroporous polymer. In certain embodiments ofthis method, at least one nucleophilic moiety on the protein is blockedprior to combining the protein with the rigid macroporous polymer.

Analyte sensors of the invention typically incorporate the porousmatrices disclosed herein. Typically, the analyte sensor apparatusincludes an analyte sensing layer disposed on a conductive layer of thesensor, typically covering a portion or all of the working electrode.This analyte sensing layer detectably alters the electrical current atthe working electrode in the conductive layer in the presence of ananalyte to be sensed. As disclosed herein, this analyte sensing layertypically includes an enzyme or antibody molecule or the like thatreacts with the analyte of interest in a manner that changes theconcentrations of a molecule that can modulate the current at theworking electrode (see e.g. oxygen and/or hydrogen peroxide as shown inthe reaction scheme of FIG. 1). Illustrative analyte sensing layerscomprise an enzyme such as glucose oxidase (e.g. for use in glucosesensors) or lactate oxidase (e.g. for use in lactate sensors). In someembodiments of the invention, the analyte sensing layer is disposed upona porous metallic and/or ceramic and/or polymeric matrix with thiscombination of elements functioning as an electrode in the sensor.

Typically, the analyte-sensing layer further comprises a carrier proteinin a substantially fixed ratio with the analyte sensing compound (e.g.the enzyme) and the analyte sensing compound and the carrier protein aredistributed in a substantially uniform manner throughout the analytesensing layer. Typically the analyte sensing layer is very thin, forexample, less than 1, 0.5, 0.25 or 0.1 microns in thickness. While notbeing bound by a specific scientific theory, it is believed that sensorshaving such thin analyte sensing layers have surprisingly enhancedcharacteristics as compared to the thicker layers that are typicallygenerated by electrodeposition because electrodeposition produces 3-5micron thick enzyme layers in which only a fraction of the reactiveenzyme within the coating layer is able to access the analyte to besensed. Such thicker glucose oxidase pellets that are produced byelectrodeposition protocols are further observed to have a poormechanical stability (e.g. a tendency to crack) and further take alonger time to prepare for actual use, typically taking weeks of testingbefore it is ready for implantation. As these problems are not observedwith the thin layered enzyme coatings described herein, these thincoatings are typical embodiments of the invention.

In sensors utilizing glucose oxidase for example, the thick coatingsproduced by electrodeposition may hinder the ability of hydrogenperoxide generated at the reactive interface of the 3-5 micron thickenzyme layer to contact the sensor surface and thereby generate asignal. In addition, hydrogen peroxide that is unable to reach a sensorsurface due to such thick coatings can diffuse away from the sensor intothe environment in which the sensor is placed, thereby decreasing thesensitivity and/or biocompatibility of such sensors. Moreover, while notbeing bound by a specific scientific theory, it is believed that sensorshaving such thin analyte sensing layers have unexpectedly advantageousproperties that result from the fact that processes such as spincoating, or the like, allow for a precise control over the enzymecoating's ratio of glucose oxidase to albumin (which is used as acarrier protein to stabilize the glucose oxidase in the enzyme layer).Specifically, because glucose oxidase and albumin have differentisoelectric points, electrodeposition processes may result in a surfacecoating in which an optimally determined ratio of enzyme to carrierprotein is detrimentally altered in the electrodeposition process, andfurther wherein the glucose oxidase and the carrier protein are notdistributed in a substantially uniform manner throughout the disposedenzyme layer. In addition, sensors having such thin analyte sensinglayers have unexpectedly faster response times. While not being bound bya specific scientific theory, it is believed that these surprising andadvantageous properties result from the observation that thin enzymelayers allow better access to the working electrode surface and mayallow a greater proportion of the molecules that modulate current at theelectrode to access the electrode surface. In this context, in certainsensor embodiments of the invention, an alteration in current inresponse to exposure to the analyte present in the body of the mammalcan be detected via an amperometer within 15, 10, 5 or 2 minutes of theanalyte contacting the analyte sensor.

Optionally, the analyte sensing layer has a protein layer disposedthereon and which is typically between this analyte sensing layer andthe analyte modulating layer. A protein within the protein layer is analbumin selected from the group consisting of bovine serum albumin andhuman serum albumin. Typically this protein is crosslinked. Withoutbeing bound by a specific scientific theory, it is believed that thisseparate protein layer enhances sensor function and provides surprisingfunctional benefits by acting as a sort of capacitor that diminishessensor noise (e.g. spurious background signals). For example, in thesensors of the invention, some amount of moisture may form under theanalyte modulating membrane layer of the sensor, the layer whichregulates the amount of analyte that can contact the enzyme of theanalyte sensing layer. This moisture may create a compressible layerthat shifts within the sensor as a patient using the sensor moves. Suchshifting of layers within the sensor may alter the way that an analytesuch as glucose moves through the analyte sensing layers in a mannerthat is independent of actual physiological analyte concentrations,thereby generating noise. In this context, the protein layer may act asa capacitor by protecting an enzyme such as GOx from contacting themoisture layer. This protein layer may confer a number of additionaladvantages such as promoting the adhesion between the analyte sensinglayer and the analyte modulating membrane layer. Alternatively, thepresence of this layer may result in a greater diffusion path formolecules such as hydrogen peroxide, thereby localizing it to theelectrode sensing element and contributing to an enhanced sensorsensitivity.

Typically, the analyte sensing layer and/or the protein layer disposedon the analyte sensing layer has an adhesion promoting layer disposedthereon. Such adhesion promoting layers promote the adhesion between theanalyte sensing layer and a proximal layer, typically an analytemodulating layer. This adhesion promoting layer typically comprises asilane compound such as γ-aminopropyltrimethoxysilane which is selectedfor its ability to promote optimized adhesion between the various sensorlayers and functions to stabilize the sensor. Interestingly, sensorshaving such a silane containing adhesion promoting layers exhibitunexpected properties including an enhanced overall stability. Inaddition, silane containing adhesion promoting layers provide a numberof advantageous characteristics in addition to an ability to enhancingsensor stability, and can, for example, play a beneficial role ininterference rejection as well as in controlling the mass transfer ofone or more desired analytes.

In certain embodiments of the invention, the adhesion promoting layerfurther comprises one or more compounds that can also be present in anadjacent layer such as the polydimethyl siloxane (PDMS) compounds thatserves to limit the diffusion of analytes such as glucose through theanalyte modulating layer. The addition of PDMS to the AP layer forexample can be advantageous in contexts where it diminishes thepossibility of holes or gaps occurring in the AP layer as the sensor ismanufactured.

Typically the adhesion promoting layer has an analyte modulating layerdisposed thereon which functions to modulate the diffusion of analytestherethrough. In one embodiment, the analyte modulating layer includescompositions (e.g. polymers and the like) which serve to enhance thediffusion of analytes (e.g. oxygen) through the sensor layers andconsequently function to enrich analyte concentrations in the analytesensing layer. Alternatively, the analyte modulating layer includescompositions which serve to limit the diffusion of analytes (e.g.glucose) through the sensor layers and consequently function to limitanalyte concentrations in the analyte sensing layer. An illustrativeexample of this is a hydrophilic glucose limiting membrane (i.e.functions to limit the diffusion of glucose therethrough) comprising apolymer such as polydimethyl siloxane or the like.

Typically the analyte modulating layer further comprises one or morecover layers which are typically electrically insulating protectivelayers disposed on at least a portion of the sensor apparatus (e.g.covering the analyte modulating layer). Acceptable polymer coatings foruse as the insulating protective cover layer can include, but are notlimited to, non-toxic biocompatible polymers such as silicone compounds,polyimides, biocompatible solder masks, epoxy acrylate copolymers, orthe like. An illustrative cover layer comprises spun on silicone.Typically the cover layer further includes an aperture that exposes atleast a portion of a sensor layer (e.g. analyte modulating layer) to asolution comprising the analyte to be sensed.

The analyte sensors described herein can be polarized cathodically todetect, for example, changes in current at the working cathode thatresult from the changes in oxygen concentration proximal to the workingcathode that occur as glucose interacts with glucose oxidase as shown inFIG. 1. Alternatively, the analyte sensors described herein can bepolarized anodically to detect for example, changes in current at theworking anode that result from the changes in hydrogen peroxideconcentration proximal to the working anode that occur as glucoseinteracts with glucose oxidase as shown in FIG. 1. In typicalembodiments of the invention, the current at the working electrode(s) iscompared to the current at a reference electrode(s) (a control), withthe differences between these measurements providing a value that canthen be correlated to the concentration of the analyte being measured.Analyte sensor designs that obtain a current value by obtaining ameasurement from a comparison of the currents at these dual electrodesare commonly termed, for example, dual oxygen sensors.

In some embodiments of the invention, the analyte sensor apparatus isdesigned to function via anodic polarization such that the alteration incurrent is detected at the anodic working electrode in the conductivelayer of the analyte sensor apparatus. Structural design features thancan be associated with anodic polarization include designing anappropriate sensor configuration comprising a working electrode which isan anode, a counter electrode which is a cathode and a referenceelectrode and then selectively disposing the appropriate analyte sensinglayer on the appropriate portion of the surface of the anode within thisdesign configuration. Optionally this anodic polarization structuraldesign includes anodes, cathodes and/or working electrodes havingdifferent sized surface areas. For example, this structural designincludes features where the working electrode (anode) and/or the coatedsurface of the working electrode is larger than the counter electrode(cathode) and/or the coated surface of the counter electrode. In thiscontext, the alteration in current that can be detected at the anodicworking electrode is then correlated with the concentration of theanalyte. In certain illustrative examples of this embodiment of theinvention, the working electrode is measuring and utilizing hydrogenperoxide in the oxidation reaction (see e.g. FIG. 1), hydrogen peroxidethat is produced by an enzyme such as glucose oxidase or lactate oxidaseupon reaction with glucose or lactate respectively. Such embodiments ofthe invention relating to electrochemical glucose and/or lactate sensorshaving such hydrogen peroxide recycling capabilities are particularlyinteresting because the recycling of this molecule reduces the amount ofhydrogen peroxide that can escape from the sensor into the environmentin which it is placed. In this context, implantable sensors that aredesigned to reduce the release of tissue irritants such as hydrogenperoxide will have improved biocompatibility profiles. Moreover as it isobserved that hydrogen peroxide can react with enzymes such as glucoseoxidase and compromise their biological function, such sensors aredesired due to their avoidance of this phenomena. Optionally, theanalyte modulating layer (e.g. a glucose limiting layer) can includecompositions that serve to inhibit the diffusion of hydrogen peroxideout in to the environment in which the sensor is placed. Consequently,such embodiments of the invention improve the biocompatibility ofsensors that incorporate enzymes that produce hydrogen peroxide byincorporating hydrogen peroxide recycling elements disclosed herein.

Certain embodiments of the analyte sensors of the invention thatcomprise a base layer, a conductive layer, an analyte sensing layer, anoptional protein layer, an adhesion promoting layer, an analytemodulating layer and a cover layer exhibit a number of unexpectedproperties. For example, in sensors that are structured to function viaanodic polarization versus those structured to function via cathodicpolarization, differences in the electrochemical reactions in theanalyte sensing layer as well as at the electrode surface generateand/or consume different chemical entities, thereby altering thechemical environment in which the various sensor elements function indifferent polarities. In this context the sensor structure disclosedherein provides a surprisingly versatile device that is shown tofunction with an unexpected degree of stability under a variety ofdifferent chemical and/or electrochemical conditions.

In certain embodiments of the invention disclosed herein (e.g., thosehaving hydrogen peroxide recycling capabilities) the sensor layer has aplurality of electrodes including a working electrode (e.g. an anode)and a counter electrode (e.g. a cathode), both of which are coated withan analyte sensing layer comprising an enzyme such as glucose oxidase orlactate oxidase. Such sensor designs have surprising propertiesincluding an enhanced sensitivity. Without being bound by a specifictheory, these properties may result from the enhanced oxidation ofhydrogen peroxide at the surface of a working or a counter electrodewhich produces additional oxygen that can be utilized in the glucosesensing reaction (see, e.g., FIG. 1). Therefore this recycling effectmay reduce the oxygen dependent limitations of certain sensorembodiments disclosed herein. Moreover, this design may result in asensor having a working electrode that can readily reduce availablehydrogen peroxide and consequently have a lower electrode potential.Sensors designed to function with lower electrode potentials are typicalembodiments of the invention because high electrode potentials insensors of this type can result in a gas producing hydrolysis reactionwhich can destabilize the sensors (due to the disruption of sensorlayers from gas bubbles produced by hydrolysis reactions). In addition,in sensor embodiments designed so that the counter electrode is coatedwith a very thin layer of an analyte sensing layer comprising an enzymesuch as glucose oxidase or lactate oxidase, the hydrogen peroxidegenerated in the enzymatic reaction is very close to the reactivesurface of the counter electrode. This can increase the overallefficiency of the sensor in a manner that allows for the production ofcompact sensor designs which include for example, counter electrodeswith smaller reactive surfaces.

A specific illustrative example of an analyte sensor apparatus forimplantation within a mammal is a peroxide sensor of the followingdesign. A first layer of the peroxide sensor apparatus is a base layer,typically made from a ceramic such as alumina. A subsequent layerdisposed upon the base layer is a conductive layer including a pluralityof electrodes including an anodic working electrode and a referenceelectrode. A subsequent layer disposed on the conductive layer is ananalyte sensing layer that includes crosslinked glucose oxidase whichsenses glucose and consequently generates hydrogen peroxide as shown inFIG. 1. In the presence of this hydrogen peroxide, the anodic workingelectrode experiences a measurable increase in current as the hydrogenperoxide generated contacts this anode in the conductive layer and isoxidized. The reference electrode serves as a control and is physicallyisolated from the working electrode and the hydrogen peroxide generatedaccording to the reaction shown in FIG. 1. This analyte sensing layer istypically less than 1, 0.5, 0.25 or 0.1 microns in thickness andcomprises a mixture of crosslinked human serum albumin in asubstantially fixed ratio with the crosslinked glucose oxidase, with theglucose oxidase and the human serum albumin being distributed in asubstantially uniform manner throughout the sensor layer. A subsequentlayer disposed on the sensor layer is a protein layer comprisingcrosslinked human serum albumin. A subsequent layer disposed on theprotein layer is an adhesion promoting layer which promotes the adhesionbetween the analyte sensing layer and/or the protein layer and ananalyte modulating layer which is disposed upon these layers. Thisadhesion promoting layer comprises a silane composition. A subsequentlayer disposed on the adhesion promoting layer is the analyte modulatinglayer in the form of a hydrophilic glucose limiting membrane comprisingPDMS which modulates the diffusion of glucose therethrough. A subsequentlayer is a cover layer, typically composed of silicone, which isdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture that exposes at least aportion of the analyte modulating layer to the external glucosecontaining environment so that the glucose can access the analytesensing layer on the working electrode. This peroxide sensor apparatusfunctions via anodic polarization such that the hydrogen peroxide signalthat is generated by glucose diffusing through the analyte modulatinglayer and then reacts with the glucose oxidase in the analyte sensinglayer creates a detectable change in the current at the anodic workingelectrode in the conductive layer of the sensor that can be measured byan amperometer. This change in the current at the anodic workingelectrode can then be correlated with the concentration of glucose inthe external environment. Consequently, a sensor of this design can actas a peroxide based glucose sensor.

E. Permutations of Analyte Sensor Apparatus and Elements

As noted above, the invention disclosed herein includes a number ofembodiments including sensors having very thin enzyme coatings. Suchembodiments of the invention allow artisans to generate a variety ofpermutations of the analyte sensor apparatus disclosed herein. As notedabove, illustrative general embodiments of the sensor disclosed hereininclude a base layer, a cover layer and at least one layer having asensor element such as an electrode disposed between the base and coverlayers. Typically, an exposed portion of one or more sensor elements(e.g., a working electrode, a counter electrode, reference electrode,etc.) is coated with a very thin layer of material having an appropriateelectrode chemistry. For example, an enzyme such as lactate oxidase,glucose oxidase, glucose dehydrogenase or hexokinase, can be disposed onthe exposed portion of the sensor element within an opening or aperturedefined in the cover layer. FIG. 2 illustrates a cross-section of atypical sensor structure 100 of the present invention. The sensor isformed from a plurality of layers of various conductive andnon-conductive constituents disposed on each other according to a methodof the invention to produce a sensor structure 100.

As noted above, in the sensors of the invention, the various layers(e.g. the analyte sensing layer) of the sensors can have one or morebioactive and/or inert materials incorporated therein. The term“incorporated” as used herein is meant to describe any state orcondition by which the material incorporated is held on the outersurface of or within a solid phase or supporting matrix of the layer.Thus, the material “incorporated” may, for example, be immobilized,physically entrapped, attached covalently to functional groups of thematrix layer(s). Furthermore, any process, reagents, additives, ormolecular linker agents which promote the “incorporation” of saidmaterial may be employed if these additional steps or agents are notdetrimental to, but are consistent with the objectives of the presentinvention. This definition applies, of course, to any of the embodimentsof the present invention in which a bioactive molecule (e.g. an enzymesuch as glucose oxidase) is “incorporated.” For example, certain layersof the sensors disclosed herein include a proteinaceous substance suchas albumin which serves as a crosslinkable matrix. As used herein, aproteinaceous substance is meant to encompass substances which aregenerally derived from proteins whether the actual substance is a nativeprotein, an inactivated protein, a denatured protein, a hydrolyzedspecies, or a derivatized product thereof. Examples of suitableproteinaceous materials include, but are not limited to enzymes such asglucose oxidase and lactate oxidase and the like, albumins (e.g. humanserum albumin, bovine serum albumin etc.), caseins, gamma-globulins,collagens and collagen derived products (e.g., fish gelatin, fish glue,animal gelatin, and animal glue).

An illustrative embodiment of the invention is shown in FIG. 2. Thisembodiment includes an electrically insulating base layer 102 to supportthe sensor 100. The electrically insulating layer base 102 can be madeof a material such as a ceramic substrate, which may be self-supportingor further supported by another material as is known in the art. In analternative embodiment, the electrically insulating layer 102 comprisesa polyimide substrate, for example a polyimide tape, dispensed from areel. Providing the layer 102 in this form can facilitate clean, highdensity mass production. Further, in some production processes usingsuch a polyimide tape, sensors 100 can be produced on both sides of thetape.

Typical embodiments of the invention include an analyte sensing layerdisposed on the base layer 102. In an illustrative embodiment as shownin FIG. 2 the analyte sensing layer comprises a conductive layer 104which is disposed on insulating base layer 102. Typically the conductivelayer 104 comprises one or more electrodes. The conductive layer 104 canbe applied using many known techniques and materials as will bedescribed hereafter, however, the electrical circuit of the sensor 100is typically defined by etching the disposed conductive layer 104 into adesired pattern of conductive paths. A typical electrical circuit forthe sensor 100 comprises two or more adjacent conductive paths withregions at a proximal end to form contact pads and regions at a distalend to form sensor electrodes. An electrically insulating protectivecover layer 106 such as a polymer coating is typically disposed onportions of the conductive layer 104. Acceptable polymer coatings foruse as the insulating protective layer 106 can include, but are notlimited to, non-toxic biocompatible polymers such as polyimide,biocompatible solder masks, epoxy acrylate copolymers, or the like.Further, these coatings can be photo-imageable to facilitatephotolithographic forming of apertures 108 through to the conductivelayer 104. In certain embodiments of the invention, an analyte sensinglayer is disposed upon a porous metallic and/or ceramic and/or polymericmatrix with this combination of elements functioning as an electrode inthe sensor.

In the sensors of the present invention, one or more exposed regions orapertures 108 can be made through the protective layer 106 to theconductive layer 104 to define the contact pads and electrodes of thesensor 100. In addition to photolithographic development, the apertures108 can be formed by a number of techniques, including laser ablation,chemical milling or etching or the like. A secondary photoresist canalso be applied to the cover layer 106 to define the regions of theprotective layer to be removed to form the apertures 108. An operatingsensor 100 typically includes a plurality of electrodes such as aworking electrode and a counter electrode electrically isolated fromeach other, however typically situated in close proximity to oneanother. Other embodiments may also include a reference electrode. Stillother embodiments may utilize a separate reference element not formed onthe sensor. The exposed electrodes and/or contact pads can also undergosecondary processing through the apertures 108, such as additionalplating processing, to prepare the surfaces and/or strengthen theconductive regions.

An analyte sensing layer 110 is typically disposed on one or more of theexposed electrodes of the conductive layer 104 through the apertures108. Typically, the analyte sensing layer 110 is a sensor chemistrylayer and most typically an enzyme layer. Typically, the analyte sensinglayer 110 comprises the enzyme glucose oxidase or the enzyme lactateoxidase. In such embodiments, the analyte sensing layer 110 reacts withglucose to produce hydrogen peroxide which modulates a current to theelectrode which can be monitored to measure an amount of glucosepresent. The sensor chemistry layer 110 can be applied over portions ofthe conductive layer or over the entire region of the conductive layer.Typically the sensor chemistry layer 110 is disposed on portions of aworking electrode and a counter electrode that comprise a conductivelayer. Some methods for generating the thin sensor chemistry layer 110include spin coating processes, dip and dry processes, low shearspraying processes, ink-jet printing processes, silk screen processesand the like. Most typically the thin sensor chemistry layer 110 isapplied using a spin coating process.

The analyte sensing layer 110 is typically coated with one or morecoating layers. In some embodiments of the invention, one such coatinglayer includes a membrane which can regulate the amount of analyte thatcan contact an enzyme of the analyte sensing layer. For example, acoating layer can comprise an analyte modulating membrane layer such asa glucose limiting membrane which regulates the amount of glucose thatcontacts the glucose oxidase enzyme layer on an electrode. Such glucoselimiting membranes can be made from a wide variety of materials known tobe suitable for such purposes, e.g., silicone, polyurethane, polyureacellulose acetate, Nafion, polyester sulfonic acid (Kodak AQ), hydrogelsor any other membrane known to those skilled in the art.

In some embodiments of the invention, a coating layer is a glucoselimiting membrane layer 112 which is disposed above the sensor chemistrylayer 110 to regulate glucose contact with the sensor chemistry layer110. In some embodiments of the invention, an adhesion promoter layer114 is disposed between the membrane layer 112 and the sensor chemistrylayer 110 as shown in FIG. 2 in order to facilitate their contact and/oradhesion. The adhesion promoter layer 114 can be made from any one of awide variety of materials known in the art to facilitate the bondingbetween such layers. Typically, the adhesion promoter layer 114comprises a silane compound. In alternative embodiments, protein or likemolecules in the sensor chemistry layer 110 can be sufficientlycrosslinked or otherwise prepared to allow the membrane layer 112 to bedisposed in direct contact with the sensor chemistry layer 110 in theabsence of an adhesion promoter layer 114.

As noted above, embodiments of the present invention can include one ormore functional coating layers. As used herein, the term “functionalcoating layer” denotes a layer that coats at least a portion of at leastone surface of a sensor, more typically substantially all of a surfaceof the sensor, and that is capable of interacting with one or moreanalytes, such as chemical compounds, cells and fragments thereof, etc.,in the environment in which the sensor is disposed. Non-limitingexamples of functional coating layers include sensor chemistry layers(e.g., enzyme layers), analyte limiting layers, biocompatible layers;layers that increase the slipperiness of the sensor; layers that promotecellular attachment to the sensor; layers that reduce cellularattachment to the sensor; and the like. Typically analyte modulatinglayers operate to prevent or restrict the diffusion of one or moreanalytes, such as glucose, through the layers. Optionally such layerscan be formed to prevent or restrict the diffusion of one type ofmolecule through the layer (e.g. glucose), while at the same timeallowing or even facilitating the diffusion of other types of moleculesthrough the layer (e.g. O₂). An illustrative functional coating layer isa hydrogel such as those disclosed in U.S. Pat. Nos. 5,786,439 and5,391,250, the disclosures of each being incorporated herein byreference. The hydrogels described therein are particularly useful witha variety of implantable devices for which it is advantageous to providea surrounding water layer.

The sensor embodiments disclosed herein can include layers havingUV-absorbing polymers. In accordance with one aspect of the presentinvention, there is provided a sensor including at least one functionalcoating layer including an UV-absorbing polymer. In some embodiments,the UV-absorbing polymer is a polyurethane, a polyurea or apolyurethane/polyurea copolymer. More typically, the selectedUV-absorbing polymer is formed from a reaction mixture including adiisocyanate, at least one diol, diamine or mixture thereof, and apolyfunctional UV-absorbing monomer.

UV-absorbing polymers are used with advantage in a variety of sensorfabrication methods, such as those described in U.S. Pat. No. 5,390,671,to Lord et al., entitled “Transcutaneous Sensor Insertion Set”; No.5,165,407, to Wilson et al., entitled “Implantable Glucose Sensor”; andU.S. Pat. No. 4,890,620, to Gough, entitled “Two-Dimensional DiffusionGlucose Substrate Sensing Electrode”, which are incorporated herein intheir entireties by reference. However, any sensor production methodwhich includes the step of forming an UV-absorbing polymer layer aboveor below a sensor element is considered to be within the scope of thepresent invention. In particular, the inventive methods are not limitedto thin-film fabrication methods, and can work with other sensorfabrication methods that utilize UV-laser cutting. Embodiments can workwith thick-film, planar or cylindrical sensors and the like, and othersensor shapes requiring laser cutting.

As disclosed herein, the sensors of the present invention areparticularly designed for use as subcutaneous or transcutaneous glucosesensors for monitoring blood glucose levels in a diabetic patient.Typically each sensor comprises a plurality of sensor elements, forexample electrically conductive elements such as elongated thin filmconductors, formed between an underlying insulative thin film base layerand an overlying insulative thin film cover layer.

If desired, a plurality of different sensor elements can be included ina single sensor. For example, both conductive and reactive sensorelements can be combined in one sensor, optionally with each sensorelement being disposed on a different portion of the base layer. One ormore control elements can also be provided. In such embodiments, thesensor can have defined in its cover layer a plurality of openings orapertures. One or more openings can also be defined in the cover layerdirectly over a portion of the base layer, in order to provide forinteraction of the base layer with one or more analytes in theenvironment in which the sensor is disposed. The base and cover layerscan be comprised of a variety of materials, typically polymers. In morespecific embodiments the base and cover layers are comprised of aninsulative material such as a polyimide. Openings are typically formedin the cover layer to expose distal end electrodes and proximal endcontact pads. In a glucose monitoring application, for example, thesensor can be placed transcutaneously so that the distal end electrodesare in contact with patient blood or extracellular fluid, and thecontact pads are disposed externally for convenient connection to amonitoring device.

The sensors of the invention can have any desired configuration, forexample planar or cylindrical. The base layer 102 can beself-supportive, such as a rigid polymeric layer, or non-selfsupportive, such as a flexible film. The latter embodiment is desirablein that it permits continuous manufacture of sensors using, for example,a roll of a polymeric film which is continuously unwound and upon whichsensor elements and coating layers are continuously applied.

A general embodiment of the invention is a sensor designed forimplantation within a body that comprises a base layer, an analytesensing layer disposed upon the base layer which includes a plurality ofsensor elements, an enzyme layer (typically less than 2 microns inthickness) disposed upon the analyte sensing layer which coats all ofthe plurality of sensing elements on the conductive layer, and one ormore coating layers. Typically the enzyme layer comprises glucoseoxidase; typically in a substantially fixed ratio with a carrierprotein. In a specific embodiment, the glucose oxidase and the carrierprotein are distributed in a substantially uniform manner throughout thedisposed enzyme layer. Typically the carrier protein comprises albumin,typically in an amount of about 5% by weight. As used herein, “albumin”refers to those albumin proteins typically used by artisans to stabilizepolypeptide compositions such as human serum albumin, bovine serumalbumin and the like. In some embodiments of the invention, a coatinglayer is an analyte contacting layer which is disposed on the sensor soas to regulate the amount of analyte that can contact the enzyme layer.In further embodiments, the sensor includes an adhesion promoter layerdisposed between the enzyme layer and the analyte contacting layer; and,the enzyme layer is less than 1, 0.5, 0.25 or 0.1 microns in thickness.

One aspect of the present invention involves processes for makingsensors having improved electrode chemistry coatings (e.g., enzymecoatings of less than 2 microns in thickness) with enhanced materialproperties. Methods for producing the extremely thin enzyme coatings ofthe invention include spin coating processes, dip and dry processes, lowshear spraying processes, ink-jet printing processes, silk screenprocesses and the like. Typically, such coatings are vapor crosslinkedsubsequent to their application. Surprisingly, sensors produced by theseprocesses have material properties that exceed those of sensors havingcoatings produced by electrodeposition including enhanced longevity,linearity, regularity as well as improved signal to noise ratios. Inaddition, certain sensor embodiments of the invention that utilizeglucose oxidase coatings formed by such processes are designed torecycle hydrogen peroxide and improve the biocompatibility profiles ofsuch sensors. Illustrative embodiments of the invention include thosedesigned to both consume hydrogen peroxide and recycle oxygen.

In this context, an illustrative embodiment of the invention is a methodof making a less than about 2 micron coating of stabilized glucoseoxidase on the surface of a matrix such as an electrode comprisingcombining glucose oxidase with albumin in a fixed ratio (one that istypically optimized for glucose oxidase stabilizing properties) andapplying the glucose oxidase and albumin mixture to the surface of thematrix by a process selected from the group consisting of a spin coatingprocess, a dip and dry process, a microdeposition process, a jet printerdeposition process, a screen printing process or a doctor bladingprocess. Typically the stabilized glucose oxidase coating is applied tothe surface of an electrode by a spin coating process. In someembodiments, the glucose oxidase/albumin is prepared in a physiologicalsolution (e.g., phosphate buffered saline at neutral pH) with thealbumin being present in an amount of about 5% albumin by weight.Optionally the stabilized glucose oxidase layer that is formed on theconductive layer is less than 2, 1, 0.5, 0.25 or 0.1 microns inthickness. A closely related embodiment of the invention is a stabilizedglucose oxidase layer for coating the surface of an electrode whereinthe glucose oxidase is mixed with a carrier protein in a fixed ratiowithin the layer, the glucose oxidase and the carrier protein aredistributed in a substantially uniform manner throughout the layer.Typically the layer is less than 2 microns in thickness.

Embodiments of the invention include a design where an analyte sensinglayer is disposed upon a porous metallic and/or ceramic and/or polymericmatrix with this combination of elements functioning as an electrode inthe sensor. A related embodiment of the invention is an electrochemicalanalyte sensor which includes a base layer, a conductive layer disposedupon the base layer that includes at least one working electrode and atleast one counter electrode, an analyte sensing layer disposed upon theconductive layer, wherein the analyte sensing layer is less than 2microns in thickness; and an analyte modulating layer that regulates theamount of analyte that contacts the enzyme layer, typically by limitingthe amount of analyte that can diffuse through the layer and contact theanalyte sensing layer. In an optional embodiment of the invention, theworking electrode and/or the coated surface of the working electrode islarger than counter electrode and/or the coated surface of the counterelectrode. In some embodiments, the enzyme layer comprises glucoseoxidase stabilized by coating it on the working electrode and thecounter electrode in combination with a carrier protein in a fixedratio. In one embodiment, this glucose oxidase enzyme layersubstantially covers the conductive layer. Embodiments where the glucoseoxidase enzyme layer is disposed in a uniform coating over the wholeconductive layer are typical because they may avoid problems associatedwith sensors having multiple different coatings on a single layer suchas the selective delamination of different coatings having differentmaterial properties. Typically, the sensor includes an adhesionpromoting layer disposed between the enzyme layer and the analytemodulating layer.

A related embodiment of the invention is an electrochemical analytesensor which includes a base layer, a conductive layer disposed upon thebase layer that includes at least one working electrode, at least onereference electrode and at least one counter electrode, an enzyme layerdisposed upon the conductive layer, and an analyte modulating coverlayer that regulates the amount of analyte that contacts the enzymelayer. In some embodiments, the enzyme layer is less than 2 microns inthickness and is coated on at least a portion of the working electrode,the reference electrode and the counter electrode. In an illustrativeembodiment, the enzyme layer substantially covers the working electrode,the reference electrode and the counter electrode. Optionally, theenzyme layer comprises glucose oxidase in combination with a carrierprotein (e.g. albumin) in a fixed ratio. Typically, the sensor includesan adhesion promoting layer disposed between the enzyme layer and theanalyte modulating layer.

Yet another embodiment of the invention comprises a glucose sensor forimplantation within a body which includes a base layer, a conductivelayer disposed upon the base layer, an analyte sensing layer comprisingglucose oxidase disposed upon the conductive layer, wherein the glucoseoxidase is stabilized by combining it with albumin in a defined ratioand further wherein the glucose oxidase and the albumin are distributedin a substantially uniform manner throughout the disposed layer, and aglucose limiting layer that regulates the amount of glucose thatdiffuses through the glucose limiting layer and contacts the glucoseoxidase layer. In some embodiments, the conductive layer includes aplurality of sensor elements including at least one working electrodeand at least one counter electrode. In such sensor embodiments, theanalyte sensing layer comprising glucose oxidase is typically less than2, 1, 0.5, 0.25 or 0.1 microns in thickness and the albumin in the layeris present in an amount of about 5% albumin by weight. Typically thesensor includes an adhesion promoting layer disposed between the analytesensing layer comprising glucose oxidase and the glucose limiting layer.

F. Analyte Sensor Apparatus Configurations

In a clinical setting, accurate and relatively fast determinations ofanalytes such as glucose and/or lactate levels can be determined fromblood samples utilizing electrochemical sensors. Conventional sensorsare fabricated to be large, comprising many serviceable parts, or small,planar-type sensors which may be more convenient in many circumstances.The term “planar” as used herein refers to the well-known procedure offabricating a substantially planar structure comprising layers ofrelatively thin materials, for example, using the well-known thick orthin-film techniques. See, for example, Liu et al., U.S. Pat. No.4,571,292, and Papadakis et al., U.S. Pat. No. 4,536,274, both of whichare incorporated herein by reference. As noted below, embodiments of theinvention disclosed herein have a wider range of geometricalconfigurations (e.g. planar) than existing sensors in the art. Inaddition, certain embodiments of the invention include one or more ofthe sensors disclosed herein coupled to another apparatus such as amedication infusion pump.

FIG. 2 provides a diagrammatic view of a typical analyte sensorconfiguration of the current invention. Certain sensor configurationsare of a relatively flat “ribbon” type configuration that can be madewith the analyte sensor apparatus. Such “ribbon” type configurationsillustrate an advantage of the sensors disclosed herein that arises dueto the spin coating of sensing enzymes such as glucose oxidase, amanufacturing step that produces extremely thin enzyme coatings thatallow for the design and production of highly flexible sensorgeometries. Such thin enzyme coated sensors provide further advantagessuch as allowing for a smaller sensor area while maintaining sensorsensitivity, a highly desirable feature for implantable devices (e.g.smaller devices are easier to implant). Consequently, sensor embodimentsof the invention that utilize very thin analyte sensing layers that canbe formed by processes such as spin coating can have a wider range ofgeometrical configurations (e.g. planar) than those sensors that utilizeenzyme layers formed via processes such as electrodeposition.

Certain sensor configurations include multiple conductive elements suchas multiple working, counter and reference electrodes. Advantages ofsuch configurations include increased surface area, which provides forgreater sensor sensitivity. For example, one sensor configurationintroduces a third working sensor. One obvious advantage of such aconfiguration is signal averaging of three sensors, which increasessensor accuracy. Other advantages include the ability to measuremultiple analytes. In particular, analyte sensor configurations thatinclude electrodes in this arrangement (e.g. multiple working, counterand reference electrodes) can be incorporated into multiple analytesensors. The measurement of multiple analytes such as oxygen, hydrogenperoxide, glucose, lactate, potassium, calcium, and any otherphysiologically relevant substance/analyte provides a number ofadvantages, for example the ability of such sensors to provide a linearresponse as well as ease in calibration and/or recalibration.

An exemplary multiple sensor device comprises a single device having afirst sensor which is polarized cathodically and designed to measure thechanges in oxygen concentration that occur at the working electrode (acathode) as a result of glucose interacting with glucose oxidase; and asecond sensor which is polarized anodically and designed to measurechanges in hydrogen peroxide concentration that occurs at the workingelectrode (an anode) as a result of glucose coming form the externalenvironment and interacting with glucose oxidase. As is known in theart, in such designs, the first oxygen sensor will typically experiencea decrease in current at the working electrode as oxygen contacts thesensor while the second hydrogen peroxide sensor will typicallyexperience an increase in current at the working electrode as thehydrogen peroxide generated as shown in FIG. 1 contacts the sensor. Inaddition, as is known in the art, an observation of the change incurrent that occurs at the working electrodes as compared to thereference electrodes in the respective sensor systems correlates to thechange in concentration of the oxygen and hydrogen peroxide moleculeswhich can then be correlated to the concentration of the glucose in theexternal environment (e.g. the body of the mammal).

The analyte sensors of the invention can be coupled with other medicaldevices such as medication infusion pumps. In a illustrative variationof this scheme, replaceable analyte sensors of the invention can becoupled with other medical devices such as medication infusion pumps,for example by the use of a port couple to the medical device (e.g. asubcutaneous port with a locking electrical connection).

II. Illustrative Methods and Materials for Making Analyte SensorApparatus of the Invention

A number of articles, U.S. patents and patent application describe thestate of the art with the common methods and materials disclosed hereinand further describe various elements (and methods for theirmanufacture) that can be used in the sensor designs disclosed herein.These include for example, U.S. Pat. Nos. 6,413,393; 6,368,274;5,786,439; 5,777,060; 5,391,250; 5,390,671; 5,165,407, 4,890,620,5,390,671, 5,390,691, 5,391,250, 5,482,473, 5,299,571, 5,568,806; UnitedStates Patent Application 20020090738; as well as PCT InternationalPublication Numbers WO 01/58348, WO 03/034902, WO 03/035117, WO03/035891, WO 03/023388, WO 03/022128, WO 03/022352, WO 03/023708, WO03/036255, WO03/036310 and WO 03/074107, the contents of each of whichare incorporated herein by reference.

Typical sensors for monitoring glucose concentration of diabetics arefurther described in Shichiri, et al.: “In Vivo Characteristics ofNeedle-Type Glucose Sensor-Measurements of Subcutaneous GlucoseConcentrations in Human Volunteers,” Horm. Metab. Res., Suppl. Ser.20:17-20 (1988); Bruckel, et al.: “In Vivo Measurement of SubcutaneousGlucose Concentrations with an Enzymatic Glucose Sensor and a WickMethod,” Klin. Wochenschr. 67:491-495 (1989); and Pickup, et al.: “InVivo Molecular Sensing in Diabetes Mellitus: An Implantable GlucoseSensor with Direct Electron Transfer,” Diabetologia 32:213-217 (1989).Other sensors are described in, for example Reach, et al., in ADVANCESIN IMPLANTABLE DEVICES, A. Turner (ed.), JAI Press, London, Chap. 1,(1993), incorporated herein by reference.

A. General Methods for Making Analyte Sensors

A typical embodiment of the invention disclosed herein is a method ofmaking a sensor apparatus for implantation within a mammal comprisingthe steps of: providing a base layer; forming a conductive layer on thebase layer, wherein the conductive layer includes an electrode (andtypically a working electrode, a reference electrode and a counterelectrode); forming an analyte sensing layer on the conductive layer,wherein the analyte sensing layer includes a composition that can alterthe electrical current at the electrode in the conductive layer in thepresence of an analyte; optionally forming a protein layer on theanalyte sensing layer; forming an adhesion promoting layer on theanalyte sensing layer or the optional protein layer; forming an analytemodulating layer disposed on the adhesion promoting layer, wherein theanalyte modulating layer includes a composition that modulates thediffusion of the analyte therethrough; and forming a cover layerdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture over at least a portion ofthe analyte modulating layer. In certain embodiments of these methods,the analyte sensor apparatus is formed in a planar geometricconfiguration

As disclosed herein, the various layers of the sensor can bemanufactured to exhibit a variety of different characteristics which canbe manipulated according to the specific design of the sensor. Forexample, the adhesion promoting layer includes a compound selected forits ability to stabilize the overall sensor structure, typically asilane composition. In some embodiments of the invention, the analytesensing layer is formed by a spin coating process and is of a thicknessselected from the group consisting of less than 1, 0.5, 0.25 and 0.1microns in height.

Typically, a method of making the sensor includes the step of forming aprotein layer on the analyte sensing layer, wherein a protein within theprotein layer is an albumin selected from the group consisting of bovineserum albumin and human serum albumin. Typically, a method of making thesensor includes the step of forming an analyte sensing layer thatcomprises an enzyme composition selected from the group consisting ofglucose oxidase, glucose dehydrogenase, lactate oxidase, hexokinase andlactate dehydrogenase. In such methods, the analyte sensing layertypically comprises a carrier protein composition in a substantiallyfixed ratio with the enzyme and the enzyme and the carrier protein aredistributed in a substantially uniform manner throughout the analytesensing layer.

B. Typical Methods for Making Porous Enzyme Matrices

One embodiment of the invention comprises porous metallic matrices.Typically, metallic substrate embodiments of the invention can bemanufactured with the desired porosity, pore-size distribution, andtortuosity through a printing process. The metallic substrate may eitherbe printed as a film or within the confines of a mold, either directlyin place onto the sensor assembly or onto a temporary substrate. The inkcan consist of fine metallic particles suspended in a porogenic carrier.The metallic particles may consist of a single pure metal or alloy.Different types of metallic particles may also be printed either at thesame time to form a mixture, or at different times to form layers. Theporogenic carrier can consist of a solvent with or without variouspolymers, glasses, ceramics, and/or frit materials. The mold may consistof various ceramics, polymers, or metals. Many thin layers of ink mayneed to be printed in order to fill the mold or to obtain a film of thedesired thickness. To remove the solvents, the printed metallic matrixcan be dried at an appropriate temperature. The resulting porous bed ofmetallic powder can then be fired approximately in the range of 350°C.-2,000° C. to bond the metallic and, if any, ceramic particlestogether. This can form a highly porous and tortuous metallic substrateonto which an enzyme such as GOx can be immobilized. If desired, themorphology of the metallic substrate can be adjusted by manipulating thesize of the metallic particles as well as the composition of theporogenic carrier. Additionally, various glass, ceramic, and/or metallicparticles included in the ink can be etched from the printed material tocreate pores using materials such as, but not limited to, hydrofluoricacid and sodium hydroxide. Prior to coating the metallic substrate withglucose oxidase, platinum black may or may not be plated using standardtechniques.

The past few years have seen increasing interest in porous metallicmaterials, especially in foams made of metals such as aluminum oraluminum alloys. Consequently, in certain embodiments of the invention,the matrix may comprise a metallic foam. Porous metals are those thatcontain a multitude of pores, i.e. closed, curved gas voids with asmooth surface. Metal(lic) foams are special cases of porous metals. Asolid foam originates from a liquid foam in which gas bubbles are finelydispersed in a liquid. In a metal sponge, space is filled by pieces ofmetal that form a continuous network and co-exist with a network ofempty space which is also interconnected. Illustrative materials of thistype are described for example in: Cellular Metals: Manufacture,Properties and Applications: J. Banhart, N. A. Fleck, A. Mortensen(Editors); and Proceedings of the 3rd International Conference onCellular Metals and Metal Foaming Technology (MetFoam 2003), J. Banhart,M. F. Ashby, N. A. Fleck (Editors), the contents of which areincorporated herein by reference.

In an alternate embodiment, the porous metallic substrates can bemanufactured by drilling small holes into a metal sheet, film, foil,rod, or block using a laser beam or some other type of drillingtechnology. In another embodiment, a woven wire mesh can be used as aporous metallic substrate. For example, the fabrication of 3-D micromeshNi Structures using electroplating has been described in the art such asfabrication methods of a 3-D micromesh Ni electrode. Specifically,inverse-micromesh photoresist structures, fabricated by multipleinclined backside exposure, can be used as a mold for Ni electroplating,with Ni meshes of about 3 μm in diameter obtained by this method.

The enzyme composition can be applied to the porous matrices by any oneof a variety of methods known in the art. In one illustrativeembodiment, an enzyme such as glucose oxidase can be dissolved in asolvent and dip, spray, or spin coated onto the porous metallicsubstrate. For some substrate geometries and morphologies, it may bedesirable to instead pump the enzyme solution through the pores. Thecoating solvent may consist of aqueous buffer and/or various organicsolvents and/or surfactants including, but not limited to, variousalcohols, dimethyl sulfoxide, and polyoxyethylene(20)sorbitanmonolaurate (“Tween™ 20”). Ingress of the protein into porous substratesmay be promoted by decreasing the viscosity of the enzyme solutionthrough the manipulation of its composition and/or by applying vacuumand/or centrifugation and/or ultrasonic vibration to the coatedsubstrate. Other bio and/or synthetic polymers may also be coated alongwith the enzyme as filler material such as, but not limited to: bovineserum albumin, human serum albumin, polyethylene glycol, andO′,O′Bis(2-aminopropyl)polyethylene glycol (“Jeffamine®”). The coatedenzyme and filler materials (if any) will be immobilized onto themetallic substrate using an appropriate homobifunctional (i.e.glutaraldehyde or disuccinimidyl suberate), heterobifunctional (i.e.succinimidyl-4-[N-maleimidomethyl]cyclohexane-1-carboxylate),trifunctional (i.e. 4-azido-2-nitrophenylbiocytin-4-nitrophenyl ester),and/or zero-length (i.e. 1-ethyl-3-[3-dimethylaminopropyl]carbodiimidehydrochloride) cross-linking agent or agents that could be selected byindividuals well versed in fields of protein immobilization,bioconjugate techniques, or polymer chemistry.

In an alternate embodiment, a process provided by SurModics Inc. underthe trademark Photolink™ can be used to immobilize an enzyme such asglucose oxidase onto the porous metallic substrate. Such Photolink™methods are set forth in U.S. Pat. Nos. 3,959,078, 4,722,906, 5,229,172;5,308,641; 5,350,800 and 5,415,938.

As disclosed herein, other embodiments of the invention include anessentially rigid, non-swelling porous enzyme-polymer matrix. In thiscontext, molded continuous rods of macroporous polymers have beendeveloped for use as chromatographic separation media (see, e.g. U.S.Pat. No. 5,453,185 and PCT Publication No. WO 93/07945, the contents ofwhich are incorporated herein by reference). Examples include, but arenot limited to poly(glycidyl methacrylate-co-ethylene dimethacrylate)and poly(styrene-co-divinylbenzene). As disclosed in U.S. Pat. No.5,453,185, a typical polymerization mixture at a minimum contains atleast one polyvinyl monomer, a free radical generating initiator, and aporogen. The mixture may also contain one or more monovinyl monomersand/or soluble polymers or insoluble macroporous polymer particles.Suitable polyvinyl monomers include divinylbenzene, divinylnaphthalene,divinylpyridine, alkylene dimethacrylates, hydroxyalkylenedimethacrylates, hydroxyalkylene diacrylates, oligoethylene glycoldimethacrylates, oligoethylene glycol diacrylates, vinyl esters ofpolycarboxylic acids, divinyl ether, pentaerythritol di-, tri-, ortetramethacrylate or acrylate, trimethylopropane trimethacrylate oracrylate, alkylene bis acrylamides or methacrylamides, and mixtures ofany such suitable polyvinyl monomers. The alkylene groups generallycontain about 1-6 carbon atoms. Monovinyl monomers which may be usedinclude styrene, ring substituted styrenes wherein the substituentsinclude chloromethyl, alkyl with up to 18 carbon atoms, hydroxyl,t-butyloxycarbonyl, halogen, nitro, amino group, protected hydroxyls oramino groups, vinylnaphthalene, acrylates, methacrylates, vinylacetate,vinylpyrolidone, and mixtures thereof. The polyvinyl monomer orpolyvinyl monomer plus the monovinyl monomer are generally present inthe polymerization mixture in an amount of from about 10 to 60 vol. %,and more typically in an amount of from about 20 to 40 vol. %. Theporogen that is used may be selected from a variety of different typesof materials. For example, suitable liquid porogens include aliphatichydrocarbons, aromatic hydrocarbons, esters, alcohols, ketones, ethers,solutions of soluble polymers, and mixtures thereof. The porogen isgenerally present in the polymerization mixture in an amount of fromabout 40 to 90 vol %, more typically from about 60 to 80 vol %. Solublepolymers and insoluble polymer particles may be employed in combinationwith the monomers. These polymers are added to the polymerizationmixture prior to polymerization. The soluble polymers are dissolved outof the plug after its formation by passing a solvent through the plug.The soluble polymers serve as a polymeric porogen to increase theporosity of the final plug. Suitable soluble polymers used hereininclude non-crosslinked polymers or copolymers of such monomers asstyrene or ring substituted styrene, acrylates, methacrylates, dienes,vinylchloride, and vinylacetate. The insoluble polymer particles areused to reduce the volume shrinkage during the polymerization. Thelesser the volume of the monomers in the polymerization mixture thesmaller the contraction of volume upon polymerization. Suitableinsoluble polymer particles used herein include macroporous polymerparticles which are cross-linked copolymers of the same monomers. It is,however, common due to compatibility to employ insoluble polymerparticles which are formed from the same monomers used to form thepolymerization mixture with which they are to be combined. The polymerparticles initially have a diameter of from about 1 to 1,000micrometers. It is not necessary that the mixture of polymer particleshave the same particle size. In fact, it is more economical, and,therefore common to use irregularly sized polymer particles. While notnecessary, the polymer particles may be soaked with a liquid immisciblewith the polymerization mixture which can contain an inhibitor whichinhibits free radical polymerization. This is done in order to preventpolymerization in the inside of the macroporous particles which wouldcause filling of the pores and would effectively remove them from theseparation process. The rod would then contain nonporous pools unable tocontribute to the separation process. Suitable inhibitors include cupricchloride and sodium nitrite. The inhibitor is generally present in anamount of from about 0.001 to 1 wt %, and more typically in an amount offrom about 0.1 to 1 wt %, based on the total weight of particles. Thepolymer particles are typically degassed prior to use in thepolymerization mixture. This may be accomplished by any of theconventional means known in the art. It, however, is typical to soak theparticles in water, optionally containing a polymerization inhibitor,and remove the air from the pores by keeping the water-polymer particlemixture under the vacuum of a water pump for a suitable period of timesuch as about 5 to 20 minutes. Excess water may then be removed byfiltering. The soluble polymers are generally present in an amount offrom about 5 to 40% by volume of the polymerization mixture and theinsoluble polymer particles in an amount of from about 5 to 50% byvolume. Conventional free-radical generating polymerization initiatorsmay be employed to initiate polymerization. Examples of suitableinitiators include peroxides such asOO-t-amyl-O-(2-ethylhexyl)monoperoxycarbonate,dipropylperoxydicarbonate, and benzoyl peroxide, as well as azocompounds such as azobisisobutyronitrile,2,2′-azobis(2-amidinopropane)dihydrochloride, and2,2′-azobis(isobutyramide)dihydrate. It has been found that the choiceof initiator may be used as a means to control the pore distribution ina plug. The initiator is generally present in the polymerization mixturein an amount of from about 0.2 to 5% by weight of the monomers.

Polymers useful for making the essentially rigid, non-swelling porousenzyme-polymer matrices are essentially incompressible and do not changetheir overall size in response to changes in their solvatingenvironment. Adjustments to the polymerization conditions can be used tocontrol the morphology of the pores. Hence, highly porous (50-70%)polymers can be created that possess significant volume fractions ofpores in the ranges of 1-100 nm and 100-3,000 nm (i.e. 20% and 80%,respectively). Polymers with this type of pore structure possess a veryhigh specific surface area (i.e. 185 m²/g), and are expected to allowfor high enzyme immobilization densities (1-100 mg/mL).

In an illustrative embodiment of the rigid, non-swelling porousenzyme-polymer matrices, a nucleophilic compound can be used tofunctionalize a macroporous, rigid polymer that possesses reactiveepoxide groups. A cross-linking agent can then be used to immobilize thebio-sensing enzyme to the polymer via the functional groups of theenzyme and polymer substrate. Other nucleophilic compounds that can beused to functionalize epoxide-activated polymers include, but are notlimited to ammonia, ethylenediamine, ethanolamine, carbohydrates,cysteine, and other amino acids. For a given enzyme and functionalizedpolymer combination, an appropriate homobifunctional (i.e.disuccinimidyl suberate), heterobifunctional (i.e.succinimidyl-4-[N-maleimidomethyl]cyclohexane-1-carboxylate),trifunctional (i.e. 4-azido-2-nitrophenylbiocytin-4-nitrophenyl ester),and/or zero-length (i.e. 1-ethyl-3-[3-dimethylaminopropyl]carbodiimidehydrochloride) cross-linking agent or agents could be selected byindividuals well versed in fields of protein immobilization orbioconjugate techniques.

In another embodiment of the rigid, non-swelling porous enzyme-polymermatrices, the bio-sensing enzyme will be directly immobilized onto anepoxide-activated polymer via nucleophilic attack by sulfhydryl, amine,hydroxyl, and/or carboxyl groups that are either native to the enzyme,or have been added to the wild-type peptide sequence via geneticengineering or directed evolution. If desired, the nucleophilicfunctional groups of the enzyme may be reversibly or irreversiblyblocked or protected during the immobilization, using compounds thatwould be familiar to anyone well versed in protein conjugation (i.e.5,5′-dithio-bis-[2-nitrobenzoic acid] or N-ethylmaleimide).

In another embodiment of the rigid, non-swelling porous enzyme-polymermatrices, monomers possessing functional groups other than (or inaddition to) epoxide groups will be incorporated into the rigid,macroporous polymer during the polymerization reaction (i.e.aminostyrene). As in this embodiment, the bio-sensing enzyme could thenbe immobilized onto the polymer substrate using an appropriatehomobifunctional, heterobifunctional, trifunctional, and/or zero-lengthcross-linking agent.

In yet another embodiment of the rigid, non-swelling porousenzyme-polymer matrices, PhotoLink® (SurModics, Eden Prairie, Minn.)chemistry can be used to immobilize the bio-sensing enzyme to themolded, porous, rigid polymer. In this embodiment, the polymer substrateneed not possess any functional groups because the PhotoLink® chemistryreacts with carbon-hydrogen groups found in virtually every organicpolymer.

C. Typical Protocols and Materials Useful in the Manufacture of AnalyteSensors

The disclosure provided herein includes sensors and sensor designs thatcan be generated using combinations of various well known techniques.The disclosure further provides methods for applying very thin enzymecoatings to these types of sensors as well as sensors produced by suchprocesses. In this context, some embodiments of the invention includemethods for making such sensors on a substrate according to art acceptedprocesses. In certain embodiments, the substrate comprises a rigid andflat structure suitable for use in photolithographic mask and etchprocesses. In this regard, the substrate typically defines an uppersurface having a high degree of uniform flatness. A polished glass platemay be used to define the smooth upper surface. Alternative substratematerials include, for example, stainless steel, aluminum, and plasticmaterials such as delrin, etc. In other embodiments, the substrate isnon-rigid and can be another layer of film or insulation that is used asa substrate, for example plastics such as polyimides and the like.

An initial step in the methods of the invention typically includes theformation of a base layer of the sensor. The base layer can be disposedon the substrate by any desired means, for example by controlled spincoating. In addition, an adhesive may be used if there is not sufficientadhesion between the substrate layer and the base layer. A base layer ofinsulative material is formed on the substrate, typically by applyingthe base layer material onto the substrate in liquid form and thereafterspinning the substrate to yield the base layer of thin, substantiallyuniform thickness. These steps are repeated to build up the base layerof sufficient thickness, followed by a sequence of photolithographicand/or chemical mask and etch steps to form the conductors discussedbelow. In an illustrative form, the base layer comprises a thin filmsheet of insulative material, such as ceramic or polyimide substrate.The base layer can comprise an alumina substrate, a polyimide substrate,a glass sheet, controlled pore glass, or a planarized plastic liquidcrystal polymer. The base layer may be derived from any materialcontaining one or more of a variety of elements including, but notlimited to, carbon, nitrogen, oxygen, silicon, sapphire, diamond,aluminum, copper, gallium, arsenic, lanthanum, neodymium, strontium,titanium, yttrium, or combinations thereof. Additionally, the substratemay be coated onto a solid support by a variety of methods well-known inthe art including chemical vapor deposition, physical vapor deposition,or spin-coating with materials such as spin glasses, chalcogenides,graphite, silicon dioxide, organic synthetic polymers, and the like.

The methods of the invention further include the generation of aconductive layer having one or more sensing elements. Typically thesesensing elements are electrodes that are formed by one of the variety ofmethods known in the art such as photoresist, etching and rinsing todefine the geometry of the active electrodes. The electrodes can then bemade electrochemically active, for example by electrodeposition of Ptblack for the working and counter electrode, and silver followed bysilver chloride on the reference electrode. A sensor layer such as asensor chemistry enzyme layer can then be disposed on the sensing layerby electrochemical deposition or a method other than electrochemicaldeposition such a spin coating, followed by vapor crosslinking, forexample with a dialdehyde (glutaraldehyde) or a carbodi-imide.

Electrodes of the invention can be formed from a wide variety ofmaterials known in the art. For example, the electrode may be made of anoble late transition metals. Metals such as gold, platinum, silver,rhodium, iridium, ruthenium, palladium, or osmium can be suitable invarious embodiments of the invention. Other compositions such as carbonor mercury can also be useful in certain sensor embodiments. Of thesemetals, silver, gold, or platinum is typically used as a referenceelectrode metal. A silver electrode which is subsequently chloridized istypically used as the reference electrode. These metals can be depositedby any means known in the art, including the plasma deposition methodcited, supra, or by an electroless method which may involve thedeposition of a metal onto a previously metallized region when thesubstrate is dipped into a solution containing a metal salt and areducing agent. The electroless method proceeds as the reducing agentdonates electrons to the conductive (metallized) surface with theconcomitant reduction of the metal salt at the conductive surface. Theresult is a layer of adsorbed metal. (For additional discussions onelectroless methods, see: Wise, E. M. Palladium: Recovery, Properties,and Uses, Academic Press, New York, N.Y. (1988); Wong, K. et al. Platingand Surface Finishing 1988, 75, 70-76; Matsuoka, M. et al. Ibid. 1988,75, 102-106; and Pearlstein, F. “Electroless Plating,” ModernElectroplating, Lowenheim, F. A., Ed., Wiley, New York, N.Y. (1974),Chapter 31.). Such a metal deposition process must yield a structurewith good metal to metal adhesion and minimal surface contamination,however, to provide a catalytic metal electrode surface with a highdensity of active sites. Such a high density of active sites is aproperty necessary for the efficient redox conversion of anelectroactive species such as hydrogen peroxide.

In an exemplary embodiment of the invention, the base layer is initiallycoated with a thin film conductive layer by electrode deposition,surface sputtering, or other suitable process step. In one embodimentthis conductive layer may be provided as a plurality of thin filmconductive layers, such as an initial chrome-based layer suitable forchemical adhesion to a polyimide base layer followed by subsequentformation of thin film gold-based and chrome-based layers in sequence.In alternative embodiments, other electrode layer conformations ormaterials can be used. The conductive layer is then covered, inaccordance with conventional photolithographic techniques, with aselected photoresist coating, and a contact mask can be applied over thephotoresist coating for suitable photoimaging. The contact masktypically includes one or more conductor trace patterns for appropriateexposure of the photoresist coating, followed by an etch step resultingin a plurality of conductive sensor traces remaining on the base layer.In an illustrative sensor construction designed for use as asubcutaneous glucose sensor, each sensor trace can include threeparallel sensor elements corresponding with three separate electrodessuch as a working electrode, a counter electrode and a referenceelectrode.

Portions of the conductive sensor layers are typically covered by aninsulative cover layer, typically of a material such as a siliconpolymer and/or a polyimide. The insulative cover layer can be applied inany desired manner. In an exemplary procedure, the insulative coverlayer is applied in a liquid layer over the sensor traces, after whichthe substrate is spun to distribute the liquid material as a thin filmoverlying the sensor traces and extending beyond the marginal edges ofthe sensor traces in sealed contact with the base layer. This liquidmaterial can then be subjected to one or more suitable radiation and/orchemical and/or heat curing steps as are known in the art. Inalternative embodiments, the liquid material can be applied using spraytechniques or any other desired means of application. Various insulativelayer materials may be used such as photoimagable epoxyacrylate, with anillustrative material comprising a photoimagable polyimide availablefrom OCG, Inc. of West Paterson, N.J., under the product number 7020.

As noted above, appropriate electrode chemistries defining the distalend electrodes can be applied to the sensor tips, optionally subsequentto exposure of the sensor tips through the openings. In an illustrativesensor embodiment having three electrodes for use as a glucose sensor,an enzyme (typically glucose oxidase) is provided within one of theopenings, thus coating one of the sensor tips to define a workingelectrode. One or both of the other electrodes can be provided with thesame coating as the working electrode. Alternatively, the other twoelectrodes can be provided with other suitable chemistries, such asother enzymes, left uncoated, or provided with chemistries to define areference electrode and a counter electrode for the electrochemicalsensor.

A significant aspect of the present invention involves processes formaking sensors having extremely thin coatings for electrode chemistries(e.g., enzyme coatings of less than 2 microns in thickness) withenhanced material properties. Methods for producing the extremely thinenzyme coatings of the invention include spin coating processes, dip anddry processes, low shear spraying processes, ink-jet printing processes,silk screen processes and the like. As artisans can readily determinethe thickness of an enzyme coat applied by process of the art, they canreadily identify those methods capable of generating the extremely thincoatings of the invention. Typically, such coatings are vaporcrosslinked subsequent to their application. Surprisingly, sensorsproduced by these processes have material properties that exceed thoseof sensors having coatings produced by electrodeposition includingenhanced longevity, linearity, regularity as well as improved signal tonoise ratios. In addition, embodiments of the invention that utilizeglucose oxidase coatings formed by such processes are designed torecycle hydrogen peroxide and improve the biocompatibility profiles ofsuch sensors.

While not being bound by a specific scientific theory, it is believedthat the surprising properties of sensors produced by such processeshave enhanced characteristics as compared to those generated byelectrodeposition because electrodeposition produces 3-5 micron thickenzyme layers in which only a fraction of the reactive enzyme is able toaccess the analyte to be sensed. Moreover, in sensors utilizing glucoseoxidase, the thick coatings produced by electrodeposition may hinder theability of hydrogen peroxide generated at the reactive interface toreach the sensor surface and thereby generate a signal. Moreover,hydrogen peroxide that is unable to reach a sensor surface due to suchthick coatings typically diffuses away from the sensor into theenvironment in which the sensor is placed, thereby decreasing thebiocompatibility of such sensors. In addition, as glucose oxidase andalbumin have different isoelectric points, electrodeposition processescan result in a surface coating in which an optimally determined ratioof enzyme to carrier protein is detrimentally altered and furtherwherein the glucose oxidase and the carrier protein are not distributedin a substantially uniform manner throughout the disposed enzyme layer.The thin coating processes utilized to produce the sensors disclosedherein avoid these problems associated with electrodeposition.

Sensors generated by processes such as spin coating processes also avoidother problems associated with electrodeposition, such as thosepertaining to the material stresses placed on the sensor during theelectrodeposition process. In particular, the process ofelectrodeposition is observed to produce mechanical stresses on thesensor, for example mechanical stresses that result from tensile and/orcompression forces. In certain contexts, such mechanical stresses mayresult in sensors having coatings with some tendency to crack ordelaminate. This is not observed in coatings disposed on sensor via spincoating or other low-stress processes. Consequently, yet anotherembodiment of the invention is a method of avoiding theelectrodeposition influenced cracking and or delamination of a coatingon a sensor comprising applying the coating via a spin coating process.

Subsequent to treatment of the sensor elements, one or more additionalfunctional coating or cover layers can then be applied by any one of awide variety of methods known in the art, such as spraying, dipping,etc. Some embodiments of the present invention include an analytemodulating layer deposited over the enzyme-containing layer. In additionto its use in modulating the amount of analyte(s) that contacts theactive sensor surface, by utilizing an analyte limiting membrane layer,the problem of sensor fouling by extraneous materials is also obviated.As is known in the art, the thickness of the analyte modulating membranelayer can influence the amount of analyte that reaches the activeenzyme. Consequently, its application is typically carried out underdefined processing conditions, and its dimensional thickness is closelycontrolled. Microfabrication of the underlying layers can be a factorwhich affects dimensional control over the analyte modulating membranelayer as well as exact the composition of the analyte limiting membranelayer material itself. In this regard, it has been discovered thatseveral types of copolymers, for example, a copolymer of a siloxane anda nonsiloxane moiety, are particularly useful. These materials can bemicrodispensed or spin-coated to a controlled thickness. Their finalarchitecture may also be designed by patterning and photolithographictechniques in conformity with the other discrete structures describedherein. Examples of these nonsiloxane-siloxane copolymers include, butare not limited to, dimethylsiloxane-alkene oxide,tetramethyldisiloxane-divinylbenzene, tetramethyldisiloxane-ethylene,dimethylsiloxane-silphenylene, dimethylsiloxane-silphenylene oxide,dimethylsiloxane-α-methylstyrene, dimethylsiloxane-bisphenol A carbonatecopolymers, or suitable combinations thereof. The percent by weight ofthe nonsiloxane component of the copolymer can be preselected to anyuseful value but typically this proportion lies in the range of about40-80 wt %. Among the copolymers listed above, thedimethylsiloxane-bisphenol A carbonate copolymer which comprises 50-55wt % of the nonsiloxane component is typical. These materials may bepurchased from Petrarch Systems, Bristol, Pa. (USA) and are described inthis company's products catalog. Other materials which may serve asanalyte limiting membrane layers include, but are not limited to,polyurethanes, cellulose acetate, cellulose nitrate, silicone rubber, orcombinations of these materials including the siloxane nonsiloxanecopolymer, where compatible.

In some embodiments of the invention, the sensor is made by methodswhich apply an analyte modulating layer that comprises a hydrophilicmembrane coating which can regulate the amount of analyte that cancontact the enzyme of the sensor layer. For example, the cover layerthat is added to the glucose sensors of the invention can comprise aglucose limiting membrane, which regulates the amount of glucose thatcontacts glucose oxidase enzyme layer on an electrode. Such glucoselimiting membranes can be made from a wide variety of materials known tobe suitable for such purposes, e.g., silicones such as polydimethylsiloxane and the like, polyurethanes, cellulose acetates, Nafion,polyester sulfonic acids (e.g. Kodak AQ), hydrogels or any othermembrane known to those skilled in the art that is suitable for suchpurposes. In certain embodiments of the invention pertaining to sensorshaving hydrogen peroxide recycling capabilities, the membrane layer thatis disposed on the glucose oxidase enzyme layer functions to inhibit therelease of hydrogen peroxide into the environment in which the sensor isplaced and to facilitate the contact between the hydrogen peroxidemolecules and the electrode sensing elements.

In some embodiments of the methods of invention, an adhesion promoterlayer is disposed between a cover layer (e.g. an analyte modulatingmembrane layer) and a sensor chemistry layer in order to facilitatetheir contact and is selected for its ability to increase the stabilityof the sensor apparatus. As noted herein, compositions of the adhesionpromoter layer are selected to provide a number of desirablecharacteristics in addition to an ability to provide sensor stability.For example, some compositions for use in the adhesion promoter layerare selected to play a role in interference rejection as well as tocontrol mass transfer of the desired analyte. The adhesion promoterlayer can be made from any one of a wide variety of materials known inthe art to facilitate the bonding between such layers and can be appliedby any one of a wide variety of methods known in the art. Typically, theadhesion promoter layer comprises a silane compound such asγ-aminopropyltrimethoxysilane. In certain embodiments of the invention,the adhesion promoting layer and/or the analyte modulating layercomprises an agent selected for its ability to crosslink a siloxanemoiety present in a proximal. In other embodiments of the invention, theadhesion promoting layer and/or the analyte modulating layer comprisesan agent selected for its ability to crosslink an amine or carboxylmoiety of a protein present in a proximal layer. In an optionalembodiment, the AP layer further comprises Polydimethyl Siloxane (PDMS),a polymer typically present in analyte modulating layers such as aglucose limiting membrane. In illustrative embodiments the formulationcomprises 0.5-20% PDMS, typically 5-15% PDMS, and most typically 10%PDMS. The addition of PDMS to the AP layer can be advantageous incontexts where it diminishes the possibility of holes or gaps occurringin the AP layer as the sensor is manufactured.

As noted above, a coupling reagent commonly used for promoting adhesionbetween sensor layers is γ-aminopropyltrimethoxysilane. The silanecompound is usually mixed with a suitable solvent to form a liquidmixture. The liquid mixture can then be applied or established on thewafer or planar sensing device by any number of ways including, but notlimited to, spin-coating, dip-coating, spray-coating, andmicrodispensing. The microdispensing process can be carried out as anautomated process in which microspots of material are dispensed atmultiple preselected areas of the device. In addition, photolithographictechniques such as “lift-off” or using a photoresist cap may be used tolocalize and define the geometry of the resulting permselective film(i.e. a film having a selective permeability). Solvents suitable for usein forming the silane mixtures include aqueous as well as water-miscibleorganic solvents, and mixtures thereof. Alcoholic water-miscible organicsolvents and aqueous mixtures thereof are particularly useful. Thesesolvent mixtures may further comprise nonionic surfactants, such aspolyethylene glycols (PEG) having a for example a molecular weight inthe range of about 200 to about 6,000. The addition of these surfactantsto the liquid mixtures, at a concentration of about 0.005 to about 0.2g/dL of the mixture, aids in planarizing the resulting thin films. Also,plasma treatment of the wafer surface prior to the application of thesilane reagent can provide a modified surface which promotes a moreplanar established layer. Water-immiscible organic solvents may also beused in preparing solutions of the silane compound. Examples of theseorganic solvents include, but are not limited to, diphenylether,benzene, toluene, methylene chloride, dichloroethane, trichloroethane,tetrachloroethane, chlorobenzene, dichlorobenzene, or mixtures thereof.When protic solvents or mixtures thereof are used, the water eventuallycauses hydrolysis of the alkoxy groups to yield organosilicon hydroxides(especially when n=1) which condense to form poly(organosiloxanes).These hydrolyzed silane reagents are also able to condense with polargroups, such as hydroxyls, which may be present on the substratesurface. When aprotic solvents are used, atmospheric moisture may besufficient to hydrolyze the alkoxy groups present initially on thesilane reagent. The R′ group of the silane compound (where n=1 or 2) ischosen to be functionally compatible with the additional layers whichare subsequently applied. The R′ group usually contains a terminal aminegroup useful for the covalent attachment of an enzyme to the substratesurface (a compound, such as glutaraldehyde, for example, may be used asa linking agent as described by Murakami, T. et al., Analytical Letters1986, 19, 1973-86).

Like certain other coating layers of the sensor, the adhesion promoterlayer can be subjected to one or more suitable radiation and/or chemicaland/or heat curing steps as are known in the art. In alternativeembodiments, the enzyme layer can be sufficiently crosslinked orotherwise prepared to allow the membrane cover layer to be disposed indirect contact with the sensor chemistry layer in the absence of anadhesion promoter layer.

An illustrative embodiment of the invention is a method of making asensor by providing a base layer, forming a sensor layer on the baselayer, spin coating an enzyme layer on the sensor layer and then formingan analyte contacting layer (e.g. an analyte modulating layer such as aglucose limiting membrane) on the sensor, wherein the analyte contactinglayer regulates the amount of analyte that can contact the enzyme layer.In some methods, the enzyme layer is vapor crosslinked on the sensorlayer. In a typical embodiment of the invention, the sensor layer isformed to include at least one working electrode and at least onecounter electrode. In certain embodiments, the enzyme layer is formed onat least a portion of the working electrode and at least a portion ofthe counter electrode. Typically, the enzyme layer that is formed on thesensor layer is less than 2, 1, 0.5, 0.25 or 0.1 microns in thickness.Typically, the enzyme layer comprises one or more enzymes such asglucose oxidase, glucose dehydrogenase, lactate oxidase, hexokinase orlactate dehydrogenase and/or like enzymes. In a specific method, theenzyme layer comprises glucose oxidase that is stabilized by coating iton the sensor layer in combination with a carrier protein in a fixedratio. Typically the carrier protein is albumin. Typically such methodsinclude the step of forming an adhesion promoter layer disposed betweenthe glucose oxidase layer and the analyte contacting layer. Optionally,the adhesion promoter layer is subjected to a curing process prior tothe formation of the analyte contacting layer.

A related embodiment of the invention is a method of making a glucosesensor by providing a base layer, forming a sensor layer on the baselayer that includes at least one working electrode and at least onecounter electrode, forming a glucose oxidase layer on the sensor layerby a spin coating process (a layer which is typically stabilized bycombining the glucose oxidase with albumin in a fixed ratio), whereinthe glucose oxidase layer coats at least a portion of the workingelectrode and at least a portion of the counter electrode, and thenforming a glucose limiting layer on the glucose sensor so as to regulatethe amount of glucose that can contact the glucose oxidase layer. Insuch processes, the glucose oxidase layer that is formed on the sensorlayer is typically less than 2, 1, 0.5, 0.25 or 0.1 microns inthickness. Typically, the glucose oxidase coating is vapor crosslinkedon the sensor layer. Optionally, the glucose oxidase coating covers theentire sensor layer. In certain embodiments of the invention, anadhesion promoter layer is disposed between the glucose oxidase layerand the analyte contacting layer. In certain embodiments of theinvention, the analyte sensor further comprises one or more cover layerswhich are typically electrically insulating protective layers (see, e.g.element 106 in FIG. 2). Typically, such cover layers are disposed on atleast a portion of the analyte modulating layer.

The finished sensors produced by such processes are typically quicklyand easily removed from a supporting substrate (if one is used), forexample, by cutting along a line surrounding each sensor on thesubstrate. The cutting step can use methods typically used in this artsuch as those that include a UV laser cutting device that is used to cutthrough the base and cover layers and the functional coating layersalong a line surrounding or circumscribing each sensor, typically in atleast slight outward spaced relation from the conductive elements sothat the sufficient interconnected base and cover layer material remainsto seal the side edges of the finished sensor. In addition, dicingtechniques typically used to cut ceramic substrates can be used with theappropriate sensor embodiments. Since the base layer is typically notphysically attached or only minimally adhered directly to the underlyingsupporting substrate, the sensors can be lifted quickly and easily fromthe supporting substrate, without significant further processing stepsor potential damage due to stresses incurred by physically pulling orpeeling attached sensors from the supporting substrate. The supportingsubstrate can thereafter be cleaned and reused, or otherwise discarded.The functional coating layer(s) can be applied either before or afterother sensor components are removed from the supporting substrate (e.g.,by cutting).

D. Cyclic Plating of Metals to Produce Biosensor Electrodes

Another illustrative embodiment of the invention is a method of making ametallic electrode useful in biosensors and electrodes made by suchmethods. These methods for making a metallic electrode includeelectrodepositing a plurality of metal layers that comprise theelectrode using cycles of differing electroplating conditions.Typically, the method comprises a first cycle of electroplating where ametal is electrodeposited onto a substrate under a first set ofconditions selected to produce a first metal layer having a firstsurface area and a first adhesion strength between the substrate and thefirst metal layer. The method then involves a second cycle ofelectroplating where a metal composition is then electrodeposited ontothe first metal layer under a second set of conditions selected toproduce a second metal layer having a second surface area and a secondadhesion strength between the first metal layer and the second metallayer. In this method, the first and second set of conditions areselected to produce a second metal layer having a second surface areathat is greater (e.g. at least 5%, 10%, 15%, 20%, 25%, or 30% greater)than the first surface area of the first metal layer produced by thefirst set of conditions and a second metal layer having an adhesion withthe first metal layer that is greater (e.g. at least 5%, 10%, 15%, 20%,25%, or 30% greater) than the adhesion between the first metal layer andthe substrate produced by the first set of conditions. Optionally, themethod further comprises additional cycles of electroplating. In onesuch example, the method comprises a third cycle of electroplating wherea metal composition is electrodeposited onto the second layer under athird set of conditions selected to produce a third metal layer having athird surface area. Typically, the second and third set of conditionsare selected to produce a third metal layer having a greater density(e.g. at least 5%, 10%, 15%, 20%, 25%, or 30% greater) than the densityof the second metal layer. In certain embodiments of the invention, thethird set of conditions produces a third metal layer having a thirdsurface area that is less than the second surface area of the secondmetal layer produced by the second set of conditions.

These embodiments of the invention can be used in a variety of contexts.In certain embodiments of the invention, the substrate does not compriseplatinum and comprises a polymer or another metal such as gold.Optionally, the substrate comprises a geometric feature selected toincrease the surface area of the electroplated metal composition. Incertain embodiments of the invention, the metal composition iselectroplated on to a porous substrate. Optionally, the substratecomprises a planar surface and an edge or lip at the boundary of theplanar surface and the cycles of differing electroplating conditionsfurther inhibit an uneven deposition of the metal layer electrodepositedonto the planar surface and the edge or lip at the boundary of theplanar surface. Typically, the uneven deposition of the metal layer thatis inhibited is a greater deposition of metal on the edge or lip at theboundary of the planar surface relative to the deposition of metal onthe planar surface. Optionally, the electroplated metal compositioncomprises platinum. Optionally, the surface area of the third layer isat least 160, 170 or 180 times the geometric area of the third layer andis typically 230-260 times the geometric area of the third layer.

A variety of protocols and conditions for making electrodes of theinvention via electroplating have been well know in the art for over 30years. For example, Feltham and Spiro in Chemical Reviews, 1971, Vol.71, No. 2, pp: 177-193 which is incorporated herein by reference teachillustrative protocols and conditions for making electrodes viaelectroplating and further teach how these conditions (e.g. currentdensity) can be controlled to affect the material properties of theelectrodeposited metal layers, for example their density and roughness(factors which correlate to surface area). Electrodeposition conditionsand the ways in which these conditions can be controlled to affect thematerial properties of electrodeposited metal layers are also describedfor example in U.S. Pat. Nos. 4,153,521, 4,280,882, 4,285,784,4,358,352, 4,427,502, 4,879,013, 4,919,768, 5,310,475, 6,139,711 and6,596,149, the contents of which are incorporated herein by reference.See also, Modern Electroplating (4th edition), M. Schlesinger and M.Paunovic (editors), Wiley, New York, 2000. Fundamentals ofElectrochemical Deposition, M. Paunovic and M. Schlesinger, Wiley, NewYork, 1998, the contents of which are incorporated herein by reference.

In certain protocols known in the art, platinum black is electroplatedonto a sensor electrode (e.g. as is used in glucose oxidase basedglucose sensors) at relatively high current densities to cause a veryrough platinum surface to develop. Using for example the recommendedplatinizing solutions and procedures described at page 193 of theFeltham and Spiro article (Chemical Reviews, 1971, Vol. 71, No. 2), at aplating current density of 186 μA/sq. mm for 120 seconds, the resultantsurface area ranges from 160 to 180 times the geometric surface area.Though high surface areas are achieved by this set of conditions, incertain circumstances, such conditions can produce electrodes havingsomewhat problematic material properties. For example, using this typeof current density protocol under controlled conditions (e.g. solutionconcentrations), excessive metal (e.g. platinum) “growth” may beobserved along the periphery of the electrode made by electrodepositinglayers of a metal(s) onto a substrate having a planar surface and theedge or lip at the boundary of the planar surface. This “growth” is forexample the result of an uneven deposition of the metal layerelectrodeposited onto the planar surface and the edge or lip at theboundary of the planar surface. In addition to this excessive growth,using this current density under controlled conditions can result in ametal layer that is fragile and further has poor adhesion with theunderlying substrate (e.g. a fragile platinum layer that exhibits amarginal adhesion to an underlying gold surface).

In illustrative embodiments of the invention using for example therecommended platinizing solutions and procedures described on page 193of the Feltham and Spiro article, cyclic plating conditions are usedthat vary the applied current density during the plating process.Specific illustrative conditions are provided in Tables 1 and 2 below.In illustrative cycles of conditions that vary the current density andtime in a defined solution of electroplated metals, a low currentdensity is first applied for 30 to 120 seconds (90 uA/sq. mm). A highercurrent is then applied for 30 to 120 seconds (140 uA/sq. mm). Finally,a low current density can be again applied for 30 to 120 seconds (90uA/sq. mm). Electrodes made by such specific combinations of cyclicplating conditions exhibit a number of surprising and beneficialmaterial properties. For example, in this illustrative embodiment, afirst low current density cycle condition results in a smooth platinumlayer, one that exhibits a strong adhesion to a substrate and acorresponding good resistance to abrasion (e.g. as measured by an artaccepted abrasion test such as a DIN, TBS or Taber abrasion testmethod). In certain embodiments of the invention, the first metal layerelectroplated under a first set of conditions therefore exhibits aresistance to abrasion from the substrate that is greater than theresistance to abrasion exhibited by a metal layer electroplated to thesubstrate under the second set of conditions. The cyclic platingconditions in this cycle therefore produce a metal layer that has thebeneficial material property of good adherence between elements (e.g.the substrate and the first metal layer). Once the first layer is“anchored” by this first set of conditions, a second higher currentdensity cycle is used to produce a further layer on this base layer onthat exhibits a roughness that is correlated with a high surface area, adesirable property for electrodes that are for example coated withenzyme layer, such as an enzyme capable of reacting with and/orproducing a molecule whose change in concentration can be measured bymeasuring a change in the current at the electrode (e.g. glucoseoxidase). At the same time, electrodes made using these first and secondconditions produce an electrodeposited metal layer where excessive“edge” growth (a factor that can lead to instability of electrodestructures) is inhibited. In certain embodiments of the invention, afurther low density cycle plating cycle is then used to plate anadditional metal layer that essentially hardens the rough layer of metalcreated in the second cycle (e.g. by making the layer more dense).Unexpectedly, electrodes made by this cycle of plating conditionsexhibit a constellation of desirable material properties including: (1)a good adhesion to the substrate (e.g. resulting from the conditions ofcycle 1); (2) a desirable amount or degree of surface area (e.g.resulting from the conditions of cycle 2); and (3) a desirable hardnessor density for final surface area of the electrode (e.g. resulting fromthe conditions of cycle 3). Alone and especially in combination, all ofthese material properties contribute to the creation of a stabilizedelectrode structure. Electrodes plated in this fashion exhibit goodadhesion to a gold substrate as well as good abrasion resistance.Furthermore, the resultant surface area ranges from 230-260 times thegeometric surface area.

Another embodiment of the invention is an implantable biosensorcomprising an electrode comprising a plurality of electrodeposited metallayers including a first metal layer having a first surface area and afirst adhesion strength with a substrate on which the first layer iselectrodeposited and a second metal layer deposited on the first metallayer, the second metal layer having a second surface area and a secondadhesion strength with the first layer on which the second layer iselectrodeposited, wherein the second surface area is greater than thefirst surface area and the second adhesion strength is greater than thefirst adhesion strength; and further an enzyme layer disposed on theelectrode, wherein an enzyme in the enzyme layer is capable of reactingwith and/or producing a molecule whose change in concentration can bemeasured by measuring a change in the current at the electrode. Incertain embodiments of the invention, the electrode comprises a thirdmetal layer deposited on the second metal layer, wherein the third metallayer has a greater density than the density of the second metal layer.Optionally the enzyme layer comprises glucose oxidase or lactateoxidase.

Yet another embodiment of the invention is an electrode comprising aplurality of electrodeposited metal layers comprising a first metallayer having a first surface area; a second metal layer deposited on thefirst metal layer, the second metal layer having a second surface areathat is greater than the first surface area of the first metal layer;and a third metal layer deposited on the second metal layer, the thirdmetal layer having a greater density than the density of the secondmetal layer. Optionally the substrate comprises a geometric featureselected to increase the surface area of the electrodeposited metalcomposition.

E. Micro-Fabricated Poly(Dimethyl Siloxane) Membrane for Use as thePermselective Sensor Layer

As noted above, certain sensor embodiments achieve their bio-specificitythrough immobilized enzymes such as glucose oxidase (GOx) or lactatedehydrogenase (LDH), which consume oxygen along with glucose or lactate(the sensor analytes) as co-reactants. To minimize the sensitivity ofthe reaction rate to the oxygen concentration in such sensors, a molarexcess of oxygen is required. However, normal physiologic conditions aresuch that glucose (˜5 mM) and lactate (˜1 mM) are almost always found inmolar excess of oxygen (˜0.05 mM). Hence, most existing sensor designsemploy a membrane that is significantly more permeable to oxygen than itis to the analyte. These permselective membranes usually containpoly(dimethylsiloxane) (PDMS), as it is biocompatible and typicallypossesses an unusually high permeability to oxygen and virtually nopermeability to analytes such as glucose or lactate. Limited analytepermeability is typically imparted upon the PDMS-based material eitherby co-polymerizing PDMS with a hydrophilic polymer (i.e. Jeffamine®) orby cutting a macroscopic “window” into a tube or sheet of PDMS.

Copolymer-type permselective membranes have successfully been used inclinically approved short-term (less than 1-week) subcutaneous glucosesensors (e.g. Continuous Glucose Monitoring Systems (“CGMS) and/or aTelemetered Glucose Monitoring Systems (“TGMS”). This type of membranesimply requires analyte to diffuse across its thickness, which isoptimal for sensor linearity and response time. However, due to the poorlong-term (i.e. 1-year) in vivo stability of hydrophilic polymers, thefeasibility of using such a permselective membrane in a long-termimplantable sensor remains in doubt. Meanwhile, macroscopic window-typepermselective membranes offer excellent long-term stability, in vivo.However, this type of membrane requires analyte to diffuse in an extramacroscopic dimension, which can negatively impact sensor linearity aswell as response time. Embodiments of the invention produce apermselective membrane fashioned from micro-fabricated PDMS thatpossesses the inherent advantages of both co-polymer and window-typebiosensor membranes. While others have constructed PDMS microstructuresthrough the casting of PDMS pre-polymers into complementarymicro-fabricated relief patterns (e.g. Kumar et al., 1994, Langmuir 10:1498-1511; Dapprich, 2003, U.S. Pat. No. 6,585,939), no one haspreviously described the use of micro-fabricated PDMS as thepermselective membrane in an enzymatic electrochemical biosensor.

In one embodiment of the invention, photolithography, lithographicmolding, thick-film printing, plasma polymerization (with or withoutshadow-masking), or discrete nano-dispensing can be used tomicro-pattern a curable PDMS functionalized derivative, co-polymer, ormixture thereof onto a pre-cast immuno-isolation membrane. Vacuum or apressure gradient may or may not be applied to promote the filling ofthe pores of the immuno-isolation membrane. Composite membranesfashioned in such a manner can possess morphologies that are layered,pore-filled, or some combination thereof.

In another embodiment of the invention, a curable PDMS derivative,co-polymer, or mixture thereof can be micro-patterned onto a temporarysubstrate using the aforementioned techniques. In the final sensorassembly, the stand-alone part may be used with a phase-inversionmembrane (“PIM”) that may either be cast as a separate part or on top ofthe PDMS (filling its pores). Various methods for promoting adhesion maybe employed by individuals skilled in the art.

In an alternate embodiment, a curable PDMS derivative, co-polymer, ormixture thereof can be micro-patterned directly onto the sensorassembly, using the techniques described above. A PIM may be placed orcast on top of the micro-patterned PDMS. Various methods for promotingadhesion may be employed by individuals skilled in the art.

In another embodiment, a laser can be used to micro-machine holes(0.1-1000 microns) into a formed piece of PDMS co-polymer (or anotherpolymeric composition such as “silicone rubber”) to form a microporousmembrane. Again, the PDMS membrane may be used with or without a PIM inthe final sensor assembly. Various methods for promoting adhesion can beemployed by individuals skilled in the art.

Illustrative chemically active groups that can be used to functionalizethe PDMS and/or PDMS co-polymer include, but are not limited to:methacrylates, acrylates, vinyls, hydrides, silanols, alkoxys, amines,epoxides, carbinols, and mercaptos. Examples of monomers that can beused to make the PDMS copolymer include, but are not limited to:phenylmethyl-, vinylmethyl-, diethyl-, methacryloxypropylmethyl-,acryloxypropylmethyl-, and alkylmethyl-siloxanes. The immuno-isolationmembrane can be pre-cast from a biocompatible polymer such aspoly(acrylonitrile-vinyl chloride) (PAN-PVC), for example using aphase-inversion process that can be optimized by individualswell-trained in the fields of biomaterials and polymer chemistry. Thecasting of the phase-inversion membrane (PIM) and the micro-patterningof the PDMS can be performed on the sensor assembly itself or on atemporary substrate such as a glass slide or silicon wafer (e.g. to forma separate part). A micro-patterned temporary substrate can also be usedto create micro-wells in the PIM into which the PDMS could be patterned.In addition, individuals skilled in the art can employ various chemicalsand techniques for promoting adhesion between the PDMS and the PIM.Examples include, but are not limited to the use of: functionalized PDMSderivatives, silanes, silane esters, functionalized silane esters,cross-linking agents, reactive polymer coatings (i.e. Lahann et al.,2003, Anal. Chem. 75: 2117-2122), plasma treatment, plasmapolymerization, shadow masking, and chemical vapor deposition.

The permselective membranes containing poly(dimethylsiloxane) provide avariety of embodiments of the invention. One embodiment of the inventionis a method of making a membrane for use with an implantable analytesensor by forming a first layer of material comprising a biocompatiblepolymer composition that is impermeable to immunoglobulins yet permeableto oxygen, glucose and lactate, and then coupling the first layer to asecond layer comprising functionalized poly(dimethyl siloxane),functionalized poly(dimethyl siloxane) copolymer or a mixture offunctionalized poly(dimethyl siloxane) and functionalized poly(dimethylsiloxane) copolymer so that a membrane for use with an implantableanalyte is made. In certain embodiments of the invention, a first layerof material comprising a biocompatible polymer composition that isimpermeable to immunoglobulins is termed a “immuno-isolation membrane”.The membrane made by this method is more permeable to oxygen than it isto compounds having a higher molecular weight such as glucose and/orlactate. Composite membranes fashioned in such a manner can be made topossess a variety of morphologies, including those that are layered,pore-filled, or some combination thereof.

Illustrative chemically active groups that can be used to functionalizethe PDMS and/or PDMS co-polymer include, but are not limited to:methacrylates, acrylates, vinyls, hydrides, silanols, alkoxys, amines,epoxides, carbinols, and mercaptos. Examples of monomers that can beused to make the PDMS copolymer include, but are not limited to:phenylmethyl-, vinylmethyl-, diethyl-, methacryloxypropylmethyl-,acryloxypropylmethyl-, and alkylmethyl-siloxanes. The other layer (e.g.the immuno-isolation membrane) can be pre-cast from a biocompatiblepolymer such as poly(acrylonitrile-vinyl chloride) (PAN-PVC), forexample using a phase-inversion process that can be optimized byindividuals well-trained in the fields of biomaterials and polymerchemistry. The casting of the phase-inversion membrane (PIM) and themicro-patterning of the PDMS can be performed on the sensor assemblyitself or on a temporary substrate such as a glass slide or siliconwafer (e.g. to form a separate part). A micro-patterned temporarysubstrate can also be used to create micro-wells in the PIM into whichthe PDMS could be patterned. While certain embodiments of the inventioninclude analyte sensors with composite membranes, the PDMS membrane maybe used with or without a PIM in the final sensor assembly.

Embodiments of these membranes can be made using a variety of well-knowntechniques. For example, in one illustrative embodiment,photolithography, lithographic molding, thick-film printing, plasmapolymerization (with or without shadow-masking), or discretenano-dispensing can be used to micro-pattern a curable PDMSfunctionalized derivative, co-polymer, or mixture thereof onto apre-cast immuno-isolation membrane. In another embodiment of theinvention, a curable PDMS derivative, co-polymer, or mixture thereof canbe micro-patterned onto a temporary substrate using the describedtechniques. In the final sensor assembly, the stand-alone part may beused with a phase-inversion membrane that may either be cast as aseparate part or on top of the PDMS (filling its pores). In an alternateembodiment, a curable PDMS derivative, co-polymer, or mixture thereofcan be micro-patterned directly onto the sensor assembly, using thetechniques described above. A PIM may be placed or cast on top of themicro-patterned PDMS.

Optionally, an adhesive layer disposed between the first and secondlayers of the membrane to promote adhesion between the first and secondlayers (as well as any other sensor layer where such an adhesive layeris appropriate). Various methods for promoting adhesion between thelayers of the membrane may be employed by individuals skilled in theart. For example, a micro-patterned temporary substrate can also be usedto create micro-wells in the PIM into which the PDMS could be patterned.In addition, individuals skilled in the art can employ various chemicalsand techniques for promoting adhesion between the PDMS and the PIM.Examples include, but are not limited to the use of functionalized PDMSderivatives, silanes, silane esters, functionalized silane esters,cross-linking agents, reactive polymer coatings (see, e.g., Lahann etal., 2003, Anal. Chem. 75: 2117-2122), plasma treatment, plasmapolymerization, shadow masking, and chemical vapor deposition.

In certain embodiments of the invention, the analyte sensor membrane caninclude additional layers having other compositions used in themanufacture of analyte sensors such as those described herein. Inaddition, in some embodiments of the invention, the first layer and/orthe second layer of the membrane is constructed to include a pluralityof pores. For example a laser can be used to micro-machine holes (e.g.of about 0.1 to about 1000 microns in size) into a formed piece of PDMSco-polymer (or another polymeric composition such as “silicone rubber”)to form a microporous membrane. In some embodiments of the invention, atleast one of the plurality of pores disposed in the second layercontains functionalized poly(dimethyl siloxane), functionalizedpoly(dimethyl siloxane) copolymer or a mixture of functionalizedpoly(dimethyl siloxane) and functionalized poly(dimethyl siloxane)copolymer of the first layer.

A related embodiment of the invention is a membrane made by thedisclosed methods. One such embodiment of the invention is a membranefor use with an implantable analyte sensor which includes a first layercomprising a biocompatible polymer composition that is impermeable toimmunoglobulins, yet permeable to oxygen, glucose and lactate; and asecond layer coupled to the first layer comprising functionalizedpoly(dimethyl siloxane), functionalized poly(dimethyl siloxane)copolymer or a mixture of functionalized poly(dimethyl siloxane) andfunctionalized poly(dimethyl siloxane) copolymer wherein the membrane ismore permeable to oxygen than glucose and/or lactate. In certainembodiments of the invention, the first and/or the second layers in themembrane comprises a plurality of pores. In certain embodiments of theinvention, an adhesive layer disposed between the first and secondlayers, wherein the adhesive layer promotes adhesion between the firstand second layers. Optionally, at least one of the plurality of poresdisposed in the second layer contains functionalized poly(dimethylsiloxane), functionalized poly(dimethyl siloxane) copolymer or a mixtureof functionalized poly(dimethyl siloxane) and functionalizedpoly(dimethyl siloxane) copolymer of the second layer. Yet anotherembodiment of the invention is an analyte sensor having a membranedisclosed above, for example an analyte sensor having a membrane madeaccording the described methods. A related embodiment is a method ofmaking an analyte sensor having such a membrane.

Another embodiment of the invention is a membrane for use with animplantable analyte sensor, the membrane including a first layercomprising a biocompatible polymer composition that is impermeable toimmunoglobulins, yet permeable to oxygen, glucose and lactate; and asecond layer coupled to the first layer comprising functionalizedpoly(dimethyl siloxane), functionalized poly(dimethyl siloxane)copolymer or a mixture of functionalized poly(dimethyl siloxane) andfunctionalized poly(dimethyl siloxane) copolymer. In this embodiment ofthe invention, the membrane is typically more permeable to oxygen thanglucose and/or lactate.

Optionally in this membrane for use with an implantable analyte sensorthe first layer and/or the second layer comprises a plurality of poresdisposed therein. In certain embodiments of the invention, at least oneof the plurality of pores disposed in the second layer containsfunctionalized poly(dimethyl siloxane), functionalized poly(dimethylsiloxane) copolymer or a mixture of functionalized poly(dimethylsiloxane) and functionalized poly(dimethyl siloxane) copolymer of thesecond layer. In some embodiments of the invention, an adhesive layercan be disposed between the first and second layers, wherein theadhesive layer promotes adhesion between the first and second layers.

A related embodiment of the invention is a method of making a membranefor use with an implantable analyte sensor generating a first layercomprising functionalized poly(dimethyl siloxane), functionalizedpoly(dimethyl siloxane) copolymer or a mixture of functionalizedpoly(dimethyl siloxane) and functionalized poly(dimethyl siloxane)copolymer; generating a second layer coupled to the first layercomprising a biocompatible polymer composition that is: impermeable toimmunoglobulins; and

permeable to oxygen, glucose and lactate so that a membrane is made thatis more permeable to oxygen than glucose and/or lactate. Optionally inthis method, the first layer and/or the second layer can be made tocomprise a plurality of pores disposed therein. In certain embodimentsof the invention, at least one of the plurality of pores disposed in thesecond layer is made to contain functionalized poly(dimethyl siloxane),functionalized poly(dimethyl siloxane) copolymer or a mixture offunctionalized poly(dimethyl siloxane) and functionalized poly(dimethylsiloxane) copolymer of the second layer. In some embodiments of themethod, an adhesive layer can be disposed between the first and secondlayers.

F. Microfabrication of Metallic Molds

A variety of methods for the microfabrication of polymerizedcompositions such as poly(dimethylsiloxane) or “PDMS” have beendeveloped in recent years and are now commonly used for the constructionof devices such as MEMS (micro-electromechanical systems) as well as inthe micro-patterning of self-assembled monolayers (e.g. “softlithography”). Typically, these methods involve the fabrication of amold that is then filled with a polymerizable composition (e.g. a PDMSpre-polymer), which is cured (polymerized) and then released to yield amicrofabricated PDMS element.

The molds used in these procedures are usually fabricated using one oftwo different approaches. In the first such approach, the negativephotoresist is coated, patterned via photolithography, and developed ona base substrate. In the second such approach, silicon wafers are etchedto form a relief pattern. In this context, the fabrication of molds withsmall, high aspect ratio features remains a significant challenge. Forexample, molds with these extreme geometries typically have poormechanical properties and can for example detach from the underlyingsubstrate during polymer release.

Mathematical modeling predicts that a layer of microporous PDMS with ahigh aspect ratio can be used as the permselective membrane of the typesused for example in enzymatic electrochemical glucose sensors. Thismathematical modeling predicts that a sensor having such a membrane willexhibit a fast, linear response to glucose. Moreover, the well-knownlong-term stability of PDMS in vivo makes a permselective membraneattractive for use in a long-term implantable sensor such as the LTGS.Clearly, the microfabrication of a mold with small, high aspect ratiofeatures that possess sufficient mechanical strength to withstand PDMSrelease is highly desirable. In this context, embodiments of theinvention disclosed herein include novel microfabrication methods thatproduce molds with mechanically robust features that are smaller in sizeand/or possess higher aspect ratios than those that can be producedthrough methods previously described in the art.

To form the layered substrate, a base substrate formed from a materialsuch as but not limited to glass, silicon, silicon nitride, or aluminumoxide is coated by a process such as sputtering with a conductivematerial such as but not limited to gold, silver, platinum, copper, orchrome. The base substrate may be pre-coated with a layer of chrome ortitanium in order to promote adhesion of the top conductive layer. Apositive photoresist such as AZ 4620 can then be applied by spin coatingor by other methods familiar to those skilled in the art ofmicrofabrication. After pre-baking, a sacrificial thin film of a metalsuch as chrome can be sputtered onto the layer of positive photo-resist.A negative photoresist such as SU-8 can be applied by spin coating or byother methods familiar to those skilled in the art. The layeredsubstrate can be obtained through standard photolithography andsubsequent development of the negative resist. Chrome etch or anotherappropriate etchant can be used to remove the areas of the sacrificialmetal layer exposed by the development of the negative resist. Thesubstrate can then be re-exposed to UV light, with or without the use ofa photomask. The positive resist can then be developed using anappropriate developing solvent, which can be selected by individualsskilled in the art. The resulting substrate can be electroplated with aconductive material such as, but not limited to gold, silver, platinum,copper, or chrome. The positive resist, sacrificial metal layer, and thenegative resist can be removed from the substrate by exposing it toacetone or another solvent that can be selected by those skilled in theart. If deemed necessary, other solvents and/or etchants such as chromeetch and negative resist strip can also be selected and applied by thoseskilled in the art. The resulting mold can be used repeatedly tomicrofabricate PDMS and other elastomers. The mold can also be used tomicrofabricate inelastic/hard materials in embodiments of the inventionwhere the electroplated material can be removed by a chemical etchant orelectrochemical oxidation.

One embodiment of the invention is a mold made by the methods describedabove. A related embodiment of the invention is a mold for forming apolymerized composition having a predetermined geometry comprising ametallic substrate capable of containing a polymerizable composition,where the polymerized composition produced by the mold has an aspectratio geometry selected to facilitate the selective diffusion ormigration of a molecule involved in the sensing process. Anotherembodiment of the invention is a mold for forming a polymerizedcomposition having a predetermined geometry comprising a metallicsubstrate capable of containing a polymerizable composition, where thepolymerized composition produced by the mold is between about 10 and2000 microns in thickness. Another embodiment of the invention is a moldfor forming a polymerized composition having a predetermined geometrycomprising a metallic substrate capable of containing a polymerizablecomposition, where the mold has sufficient mechanical strength towithstand release of a polymerized poly(dimethylsiloxane) compositionwithout breaking. In certain embodiments of the invention, the mold hasa two or more of these features.

One embodiment of the invention is a method of making a mold for forminga polymerized composition of a predetermined geometry by providing abase substrate; disposing a conductive layer on to (at least a portion)the base substrate; disposing a positive photoresist layer on to theconductive layer; disposing a sacrificial metal layer on to the positivephotoresist layer; disposing a negative photoresist layer on to thesacrificial metal layer; developing the negative photoresist layer viaUV photolithography (with or without the use of a photomask); removingthe areas of the sacrificial metal layer exposed by the development ofthe negative resist layer using an etchant; exposing these components toUV photolithography; developing the positive photoresist layer via adeveloping solvent; electroplating these components with a layer ofconductive material; and removing the positive photoresist layer, thesacrificial metal layer, and the negative photoresist layer from the solayered substrate using a solvent so that the mold is made. Typically,the mold made by the method can be used repeatedly. Other embodiments ofthe invention include a polymerized composition layer made using thedescribed molds as well as analyte sensor including a polymerizedcomposition layer made using the described molds.

In embodiments of the invention, the base substrate can be formed from awide variety of materials such as glass, silicon, silicon nitride,aluminum oxide or the like. In certain embodiments of the invention theconductive material is disposed on the base by a process such assputtering. Conductive materials for use in embodiments of the inventioninclude gold, silver, platinum, copper, chrome or the like. In certainembodiments of the invention, the base substrate is coated with a layerof chrome or titanium prior to the application of the conductivematerial in order to promote adhesion of the base substrate and theconductive layer. In some embodiments of the invention, the substrate isbaked prior to disposing the sacrificial metal layer on to the positivephotoresist layer. Optionally in these methods, the negative photoresistlayer and/or the positive photoresist layer is applied to the substrateby spin coating.

One embodiment of the invention is a mold for forming a polymerizedcomposition having a predetermined geometry comprising a metallicsubstrate capable of containing a polymerizable composition; where thepolymerized composition produced by the mold has an optimized aspectratio geometry, the polymerized composition produced by the mold isbetween about 10 to 2000 microns in thickness; and the mold hassufficient mechanical strength to withstand release of a polymerizedpoly(dimethylsiloxane) composition without breaking. A relatedembodiment of the invention is a method of making a mold for forming apolymerized composition of a predetermined geometry comprising:providing a base substrate; disposing a conductive layer on to (aportion) the base substrate; disposing a positive photoresist layer onto the conductive layer; disposing a sacrificial metal layer on to thepositive photoresist layer; disposing a negative photoresist layer on tothe sacrificial metal layer; developing the negative photoresist layervia UV photolithography; removing the areas of the sacrificial metallayer exposed by the development of the negative resist layer using anetchant; exposing these components to UV photolithography (with orwithout the use of a photomask); developing the positive photoresistlayer via a developing solvent; electroplating these components with alayer of conductive material; and then removing the positive photoresistlayer, the sacrificial metal layer, and the negative photoresist layerfrom the so layered substrate using a solvent so that the mold is made.Typically, the mold made by the method can be used repeatedly.Optionally in this method, the base substrate is coated with a layer ofchrome or titanium prior to the application of the conductive layer inorder to promote adhesion of the base substrate and the conductivelayer. Optionally, the substrate is baked prior to disposing thesacrificial metal layer on to the positive photoresist layer. Optionallythe negative photoresist layer and/or the positive photoresist layer isapplied to the substrate by spin coating.

III. Methods for Using Analyte Sensor Apparatus of the Invention

A related embodiment of the invention is a method of sensing an analytewithin the body of a mammal, the method comprising implanting an analytesensor embodiment disclosed herein in to the mammal and then sensing analteration in current at the working electrode and correlating thealteration in current with the presence of the analyte, so that theanalyte is sensed. Typically the analyte sensor is polarized anodicallysuch that the working electrode where the alteration in current issensed is an anode. In one such method, the analyte sensor apparatussenses glucose in the mammal. In an alternative method, the analytesensor apparatus senses lactate, potassium, calcium, oxygen, pH, and/orany physiologically relevant analyte in the mammal.

Certain analyte sensors having the structure discussed above have anumber of highly desirable characteristics which allow for a variety ofmethods for sensing analytes in a mammal. For example in such methods,the analyte sensor apparatus implanted in the mammal functions to sensean analyte within the body of a mammal for more than 1, 2, 3, 4, 5, or 6months. Typically, the analyte sensor apparatus so implanted in themammal senses an alteration in current in response to an analyte within15, 10, 5 or 2 minutes of the analyte contacting the sensor. In suchmethods, the sensors can be implanted into a variety of locations withinthe body of the mammal, for example in both vascular and non-vascularspaces.

IV. Kits and Sensor Sets of the Invention

In another embodiment of the invention, a kit and/or sensor set, usefulfor the sensing an analyte as is described above, is provided. The kitand/or sensor set typically comprises a container, a label and ananalyte sensor as described above. Suitable containers include, forexample, an easy to open package made from a material such as a metalfoil, bottles, vials, syringes, and test tubes. The containers may beformed from a variety of materials such as metals (e.g. foils) paperproducts, glass or plastic. The label on, or associated with, thecontainer indicates that the sensor is used for assaying the analyte ofchoice. In some embodiments, the container holds a porous matrix that iscoated with a layer of an enzyme such as glucose oxidase. The kit and/orsensor set may further include other materials desirable from acommercial and user standpoint, including elements or devices designedto facilitate the introduction of the sensor into the analyteenvironment, other buffers, diluents, filters, needles, syringes, andpackage inserts with instructions for use.

Various citations are referenced throughout the specification. Inaddition, certain text from related art is reproduced herein to moreclearly delineate the various embodiments of the invention. Thedisclosures of all citations in the specification are expresslyincorporated herein by reference.

TABLE 1 Lot 3422 Lot 3422 Lot 3422 Lot 3422 Lot 3422 Plates 1-2 Plates3-4 Plates 5-6 Plates 7-8 Plates 9-10 2x 3x 2x 3x 2x3x 2x 3x 2x 3xStandard metal Cyclic metal Cyclic metal Cyclic metal Cyclic metalplating for 2x3x plating plating plating plating electrodes. (100% 120sec, −36 uA, 120 sec, −36 uA, 60 sec, −36 uA, 60 sec, −36 uA, currentdensity) 160 sec, −54 uA, 120 160 sec, −54 uA, 60 160 sec, 54 uA, 120160 sec, −54 uA, 60 sec, −36 uA just for sec, −36 uA just for sec, −36uA just for sec, −36 uA just for working electrode. working electrode.working electrode. working electrode. Counter and ref Counter and refCounter and ref Counter and ref electrodes get std electrodes get stdelectrodes get std electrodes get std plating for 2x3x plating for 2x3xplating for 2x3x plating for 2x3x config. config. config. config. Plasma200 W, 15 s Plasma 200 W, 15 s Plasma 200 W, 15 s Plasma 200 W, 15 sPlasma 200 W, 15 s spin Gox at 500 rpm spin Gox at 500 rpm spin Gox at500 rpm spin Gox at 500 rpm spin Gox at 500 rpm x-link 2 hr, room x-link2 hr., room x-link 2 hr., room x-link 2 hr., room x-link 2 hr., roomtemperature, rinse temperature, rinse temperature, rinse temperature,rinse temperature, rinse 30 min 30 min 30 min 30 min 30 min spin AP10%-spin AP10%- spin AP10%- spin AP10%- spin AP10%- Ethanol no water,Ethanol no water, Ethanol no water, Ethanol no water, Ethanol no water,2000 rpm 2000 rpm 2000 rpm 2000 rpm 2000 rpm Room temperature Roomtemperature Room temperature Room temperature Room temperature glut -2.5hr glut -2.5 hr glut -2.5 hr glut -2.5 hr glut -2.5 hr rinse 30 minrinse 30 min rinse 30 min rinse 30 min rinse 30 min Test 2 sensors perTest 2 sensors per Test 2 sensors per Test 2 sensors per Test 2 sensorsper plate, then surface plate, then surface plate, then surface plate,then surface plate, then surface area test area test area test area testarea test Standard GLM Standard GLM Standard GLM Standard GLM StandardGLM 169 kD at 0.9 yos, 169 kD at 0.9 yos, 169 kD at 0.9 yos, 169 kD at0.9 yos, 169 kD at 0.9 yos, 31 rpts 31 rpts 31 rpts 31 rpts 31 rpts NOHPM NO HPM NO HPM NO HPM NO HPM Test 1 sensor per Test 1 sensor per Test1 sensor per Test 1 sensor per Test 1 sensor per plate plate plate plateplate Assemble as -003 Assemble as -003 Assemble as -003 Assemble as-003 Assemble as -003 and sterilize and sterilize and sterilize andsterilize and sterilize

TABLE 2 Lot 3422 Lot 3422 Lot 3422 Plates 11-22 Plates 13-14 Plates15-16 2x 3x-Control 2x3x 2x 3x Standard metal Cyclic metal Cyclic metalplating for 2x3x plating plating electroces. (100% 60 sec @ −36 uA, 60sec @ −36 uA, current density) 120 sec @ −58 uA, 120 sec @ −58 uA, 120sec @ −36 uA 120 sec @ −36 uA, for working for working electrode.Counter electrode. Plate and ref electrodes Counter for 60 sec get stdplating for @ 72 uA, 120 sec 2x3x config. @ 96 uA and 120 sec @ 72 uA.Ref electrodes get std plating. Plasma 200 W, 15 s Plasma 200 W, 15 sPlasma 200 W, 15 s spin Gox at 500 rpm spin Gox at 500 rpm spin Gox at500 rpm x-link 2 hr, room x-link 2 hr, room x-link 2 hr, roomtemperature, rinse temperature, rinse temperature, rinse 30 min 30 min30 min spin adhesion spin AP10%- spin AP10%- promoter (AP) 10%- Ethanolno water, Ethanol no water, Ethanol no water, 2000 rpm 2000 rpm 2000 rpmRoom temperature Room temperature Room temperature glutaraldehyde -2.5hr glut -2.5 hr glut -2.5 hr rinse 30 min rinse 30 min rinse 30 min Test2 sensors per Test 2 sensors per Test 2 sensors per plate, then surfaceplate, surface plate, then surface area test area test area testStandard GLM Standard GLM Standard GLM 169 kD at 1.0 yos, 169 kD at 1.0yos, 169 kD at 1.0 yos, 31 rpts 31 rpts 31 rpts NO HPM NO HPM NO HPMTest 1 sensor per Test 1 sensor per Test 1 sensor per plate plate plateAssemble as -003 Assemble as -003 Assemble as -003 and sterilize andsterilize and sterilize

1. A method of modulating electrochemical reactions within animplantable analyte sensor, the method comprising performingelectrochemical reactions within an implantable analyte sensorcomprising: a working electrode having a reactive surface area, whereinduring analyte sensing, the working electrode generates electrons thatreduce a plurality of composition species in the electrochemicalreaction including oxygen (O₂); and a counter electrode having areactive surface area, wherein the size of the reactive surface area ofthe counter electrode is selected so as to control the reduction of theplurality of composition species in the electrochemical reaction so thatoxygen (O₂) is the predominant composition species reduced by theelectrons generated at the working electrode; so that electrochemicalreactions within the implantable analyte sensor are modulated.
 2. Themethod of claim 1, wherein the surface area of the counter electrode is1.5, 2, 2.5 or 3 times the size of the working electrode.
 3. The methodof claim 1, wherein the implantable analyte sensor comprises an analytesensing layer disposed on the working electrode, wherein the analytesensing layer detectably alters the electrical current at the workingelectrode in the presence of an analyte; an optional protein layerdisposed on the analyte sensing layer; an adhesion promoting layerdisposed on the analyte sensing layer or the optional protein layer,wherein the adhesion promoting layer promotes the adhesion between theanalyte sensing layer and an analyte modulating layer disposed on theanalyte sensing layer; and an analyte modulating layer disposed on theanalyte sensing layer, wherein the analyte modulating layer modulatesthe diffusion of the analyte therethrough; and an optional cover layerdisposed on at least a portion of the analyte modulating layer, whereinthe cover layer further includes an aperture over at least a portion ofthe analyte modulating layer.
 4. The method of claim 1, wherein theworking electrode and the counter electrode comprise a micro-porousmatrix.
 5. The method of claim 4, wherein the micro-porous matrix has asurface area that is at least 2, 4, 6, 8, 10, 12, 14, 16 or 18 times thesurface area of a non-porous matrix of same dimensions.
 6. The method ofclaim 2, wherein the analyte sensing layer is a protein.
 7. The methodof claim 6, wherein the protein is glucose oxidase, glucosedehydrogenase, lactate oxidase, hexokinase or lactate dehydrogenase. 8.The method of claim 1, wherein hydrogen peroxide is oxidized at theworking electrode.
 9. The method of claim 2, wherein the implantableanalyte sensor further comprises an interference rejection layerdisposed between the surface of the working electrode and the analytesensing layer.
 10. An implantable electrochemical analyte sensorcomprising: a working electrode having a reactive surface area, whereinduring analyte sensing, the working electrode generates electrons thatreduce a plurality of composition species in the electrochemicalreaction including oxygen (O₂); and a counter electrode having areactive surface area, wherein the size of the reactive surface area ofthe counter electrode is selected so as to control the reduction of theplurality of composition species in the electrochemical reaction so thatoxygen (O₂) is the predominant composition species reduced by theelectrons generated at the working electrode; an analyte sensing layerdisposed on the working electrode, wherein the analyte sensing layerdetectably alters the electrical current at the working electrode in thepresence of an analyte; an adhesion promoting layer disposed on theanalyte sensing layer or the protein layer, wherein the adhesionpromoting layer promotes the adhesion between the analyte sensing layerand an analyte modulating layer disposed on the analyte sensing layer;and an analyte modulating layer disposed on the analyte sensing layer,wherein the analyte modulating layer modulates the diffusion of theanalyte therethrough; and a cover layer disposed on at least a portionof the analyte modulating layer, wherein the cover layer furtherincludes an aperture over at least a portion of the analyte modulatinglayer.